docs: add production sync testing guide and log sanitizer script

Add infrastructure for iterative debugging of sync against production data:
- scripts/sanitize-logs.sh: Truncates verbose log lines for LLM analysis
- docs/how-to/production-sync-testing.md: Step-by-step guide for testing
  sync against real relays, identifying issues, and improving logging

2 files changed, 295 insertions, 0 deletions

diff --git a/docs/how-to/README.md b/docs/how-to/README.md
index ed5f014..ba58c08 100644
--- a/docs/how-to/README.md
+++ b/docs/how-to/README.md
@@ -36,6 +36,18 @@ How-to guides are **recipes** that show you how to solve specific problems or ac
 
 ---
 
+### [Test Sync Against Production Data](production-sync-testing.md)
+**Problem:** Debug and improve sync using real-world data  
+**Difficulty:** Intermediate
+
+**You'll learn:**
+- Run sync against production relays
+- Sanitize logs for LLM analysis
+- Identify common issues and patterns
+- Iteratively improve sync behavior
+
+---
+
 ## Planned How-To Guides
 
 ### Deploy ngit-grasp
diff --git a/docs/how-to/production-sync-testing.md b/docs/how-to/production-sync-testing.md
new file mode 100644
index 0000000..dfff20d
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+++ b/docs/how-to/production-sync-testing.md
@@ -0,0 +1,283 @@
+# How-To: Test Sync Against Production Data
+
+**Problem:** Debug and improve sync behavior using real-world data from production relays  
+**Difficulty:** Intermediate  
+**Time:** 30 minutes per iteration
+
+## Overview
+
+This guide helps you run ngit-grasp's sync system against production relays to discover unexpected errors, inefficiencies, and edge cases that don't appear in controlled tests.
+
+**Why production testing matters:**
+- Real data has inconsistencies, malformed events, and edge cases
+- Production relays may behave differently (rate limiting, timeouts, partial NIP-77 support)
+- Volume and patterns reveal performance bottlenecks
+- Sync discovery leads to cascading subscriptions we can't predict in tests
+
+## Prerequisites
+
+- ngit-grasp compiles successfully (`cargo build`)
+- Familiarity with [GRASP-02 Proactive Sync](../explanation/grasp-02-proactive-sync.md)
+- Understanding of log levels and tracing
+
+## Test Setup
+
+### 1. Choose a Test Identity
+
+Pick a domain with manageable sync volume. Smaller domains mean fewer repos to sync, making logs tractable.
+
+**Recommended starting point:**
+```bash
+--domain ngit.danconwaydev.com
+```
+
+This domain has few repo announcements listing it, so sync stays manageable.
+
+### 2. Choose a Bootstrap Relay
+
+The bootstrap relay provides the initial set of announcements to discover repos:
+
+```bash
+--sync-bootstrap-relay wss://git.shakespeare.diy
+```
+
+### 3. Run with Time Limit
+
+Start with short runs (30 seconds) to capture manageable log volumes:
+
+```bash
+# Clear any existing data for clean state
+rm -rf /tmp/ngit-test-*
+
+# Run for 30 seconds with sanitized output
+timeout 30s cargo run -- \
+    --sync-bootstrap-relay wss://git.shakespeare.diy \
+    --domain ngit.danconwaydev.com \
+    --git-path /tmp/ngit-test-git \
+    --relay-data-path /tmp/ngit-test-relay \
+    2>&1 | ./scripts/sanitize-logs.sh | tee sync-test.log
+```
+
+**Note:** The `timeout` command returns exit code 124, which is expected.
+
+## Log Sanitization
+
+Raw logs include full events and hundreds of event IDs per line, making them unwieldy for analysis. The sanitizer truncates long lines:
+
+```bash
+./scripts/sanitize-logs.sh < raw.log > sanitized.log
+
+# Or pipe directly
+cargo run -- [args] 2>&1 | ./scripts/sanitize-logs.sh
+```
+
+**Options:**
+- `--head-chars N` - First N characters to show (default: 100)
+- `--tail-chars N` - Last N characters to show (default: 20)
+
+Example output:
+```
+2024-01-09T10:00:00Z DEBUG sync: Processing events ids=[abc123, def456, ghi789, jkl012...<1847 chars>...xyz999, end123]
+```
+
+## What to Look For
+
+### Phase 1: Connection & Bootstrap (0-5 seconds)
+
+**Expected behavior:**
+- Connection to bootstrap relay succeeds
+- Layer 1 (announcement) subscription starts
+- First batch of 30617/30618 events received
+
+**Red flags:**
+- Connection timeout or failure
+- NIP-77 negentropy errors (should fall back gracefully)
+- Immediate rate limiting
+
+### Phase 2: Discovery Cascade (5-15 seconds)
+
+**Expected behavior:**
+- Self-subscriber batches fire as announcements are processed
+- New relays discovered from announcement `relays` tags
+- Layer 2 (repo tags) subscriptions created
+
+**Red flags:**
+- Excessive relay discovery (>10 relays rapidly)
+- Filter consolidation warnings (>70 filters)
+- Missing self-subscriber batch logs
+
+### Phase 3: Steady State (15+ seconds)
+
+**Expected behavior:**
+- Historic sync batches completing (EOSE received)
+- Periodic health checks running
+- Events being saved to database
+
+**Red flags:**
+- Pending batches never confirming
+- Repeated connection/disconnect cycles
+- Memory growth (check with `top` in another terminal)
+
+## Debugging Checklist
+
+When analyzing logs, look for these patterns:
+
+### Errors to Investigate
+
+| Pattern | Possible Cause | Action |
+|---------|----------------|--------|
+| `error` (any) | Unexpected failure | Investigate immediately |
+| `connection failed` | Network/relay issue | Check relay URL, try different relay |
+| `rate limit` | Too many requests | Check consolidation, increase backoff |
+| `negentropy` + `error` | NIP-77 incompatibility | Should fall back - verify it does |
+| `timeout` | Slow relay or large sync | Increase timeouts or reduce scope |
+
+### Warnings to Monitor
+
+| Pattern | Meaning | Action |
+|---------|---------|--------|
+| `consolidating filters` | Filter count high | Expected, but frequent = problem |
+| `backing off` | Health tracker retry | Normal, but watch for excessive |
+| `batch failed` | Historic sync incomplete | Check which batches, why |
+
+### Debug Patterns to Verify
+
+| Pattern | What it shows |
+|---------|---------------|
+| `fresh_start` | Full sync initiated |
+| `quick_reconnect` | Incremental sync (<15min gap) |
+| `historic sync complete` | Sync finished successfully |
+| `sync_live` | Live subscriptions active |
+| `PendingBatch` | Items awaiting EOSE confirmation |
+
+## Iterative Improvement Process
+
+### Step 1: Run and Capture
+
+```bash
+timeout 30s cargo run -- [args] 2>&1 | ./scripts/sanitize-logs.sh > iteration-1.log
+```
+
+### Step 2: Identify Issues
+
+Scan logs for errors and unexpected patterns:
+```bash
+grep -i error iteration-1.log
+grep -i warn iteration-1.log
+grep -i panic iteration-1.log
+```
+
+### Step 3: Document Findings
+
+Add findings to this file's [Known Issues](#known-issues) section or create GitHub issues.
+
+### Step 4: Fix and Re-test
+
+After code changes, run again to verify the fix.
+
+### Step 5: Extend Duration
+
+Once 30-second runs are clean, extend to 2 minutes, then 5 minutes:
+```bash
+timeout 120s cargo run -- [args] 2>&1 | ./scripts/sanitize-logs.sh > iteration-2.log
+```
+
+## Logging Improvements
+
+If the logs aren't helpful enough, improve them. Common needs:
+
+### Add Context to Existing Logs
+
+```rust
+// Before
+tracing::debug!("Processing events");
+
+// After
+tracing::debug!(
+    relay = %relay_url,
+    event_count = events.len(),
+    "Processing events"
+);
+```
+
+### Add New Log Points
+
+Key places that may need more logging:
+- `src/sync/mod.rs` - SyncManager state transitions
+- `src/sync/relay_connection.rs` - Connection lifecycle
+- `src/sync/self_subscriber.rs` - Batch processing
+
+### Reduce Noise
+
+If a log line appears too frequently:
+```rust
+// Change from debug! to trace!
+tracing::trace!("Per-event detail that's too noisy");
+```
+
+## Known Issues
+
+*Document issues discovered during testing here. Delete this section when empty.*
+
+### Template for New Issues
+
+```markdown
+### Issue: [Short description]
+
+**Discovered:** [Date]
+**Status:** [Open/Fixed in PR#xxx]
+
+**Symptoms:**
+- Log pattern observed
+
+**Root cause:**
+- [If known]
+
+**Fix:**
+- [If known]
+```
+
+---
+
+## Quick Reference
+
+### Minimal Test Command
+
+```bash
+timeout 30s cargo run -- \
+    --sync-bootstrap-relay wss://git.shakespeare.diy \
+    --domain ngit.danconwaydev.com \
+    --git-path /tmp/ngit-test-git \
+    --relay-data-path /tmp/ngit-test-relay \
+    2>&1 | ./scripts/sanitize-logs.sh
+```
+
+### With Metrics Endpoint
+
+```bash
+timeout 30s cargo run -- \
+    --sync-bootstrap-relay wss://git.shakespeare.diy \
+    --domain ngit.danconwaydev.com \
+    --git-path /tmp/ngit-test-git \
+    --relay-data-path /tmp/ngit-test-relay \
+    --metrics-address 127.0.0.1:9090 \
+    2>&1 | ./scripts/sanitize-logs.sh
+```
+
+Then in another terminal: `curl http://127.0.0.1:9090/metrics`
+
+### Different Log Level
+
+The default is DEBUG. For more detail:
+```bash
+RUST_LOG=trace cargo run -- [args]
+```
+
+For less noise:
+```bash
+RUST_LOG=info cargo run -- [args]
+```
+
+---
+
+*Part of the [ngit-grasp documentation](../README.md) using the [Diátaxis](https://diataxis.fr/) framework.*