feat: Switch SyncManager to use two-tier RejectedEventsIndex

Replaces the simple HashSet<EventId> with the sophisticated two-tier
RejectedEventsIndex from PR1, enabling future immediate re-processing
when maintainer dependencies resolve.

## Changes

### Config (src/config.rs)
- Add `rejected_hot_cache_duration_secs` (default: 120 = 2 minutes)
- Add `rejected_cold_index_expiry_secs` (default: 604800 = 7 days)
- Both configurable via CLI flags or environment variables

### SyncManager (src/sync/mod.rs)

**Type Change:**
- Before: `Arc<RwLock<HashSet<EventId>>>` (simple event ID set)
- After: `Arc<RejectedEventsIndex>` (two-tier storage)

**Initialization:**
- Pass config durations to RejectedEventsIndex::new()
- Creates hot cache (2 min) + cold index (7 days)

**Event Processing (process_event_static):**
- Extract identifier from 'd' tag
- Determine rejection reason from error message
- Call `add_announcement()` with full event + metadata
- Stores in both hot cache and cold index

**Negentropy Sync (derive_relay_targets):**
- Call `get_all_event_ids()` to get rejected IDs
- Returns union of hot cache + cold index event IDs
- Excludes from negentropy reconciliation

**Event Loop (relay_connection):**
- Use `contains()` method instead of direct HashSet access
- Simpler API, same skip-rejected behavior

### RejectedEventsIndex (src/sync/rejected_index.rs)

**New Method:**
- `get_all_event_ids()`: Returns HashSet<EventId> from both tiers
- Used for negentropy exclusion (replaces direct HashSet access)

### Tests Updated

**test_rejected_events_index_tracks_announcements:**
- Create RejectedEventsIndex with config durations
- Add 'd' tag to test announcement
- Use `add_announcement()` with full event
- Verify both hot cache and cold index populated
- Check lengths with `hot_cache_len()` and `cold_index_len()`

**test_rejected_events_excluded_from_negentropy:**
- Create RejectedEventsIndex instead of HashSet
- Build full event with 'd' tag
- Add to index with `add_announcement()`
- Get IDs with `get_all_event_ids()`
- Verify excluded from reconciliation

## Architecture

```
┌─────────────────────────────────────────────────────────────┐
│ SyncManager                                                  │
│                                                              │
│  rejected_events_index: Arc<RejectedEventsIndex>            │
│  ├─ Hot Cache (2 min): Full events for re-processing        │
│  └─ Cold Index (7 days): Metadata for dedup                 │
└─────────────────────────────────────────────────────────────┘
                        │
                        │ On rejection
                        ▼
┌─────────────────────────────────────────────────────────────┐
│ add_announcement(event, pubkey, identifier, reason)         │
│  ├─ Store full event in hot cache                           │
│  └─ Store metadata in cold index                            │
└─────────────────────────────────────────────────────────────┘
                        │
                        │ On negentropy sync
                        ▼
┌─────────────────────────────────────────────────────────────┐
│ get_all_event_ids() → HashSet<EventId>                      │
│  ├─ Union of hot cache IDs                                  │
│  └─ Union of cold index IDs                                 │
└─────────────────────────────────────────────────────────────┘
```

## Benefits

### Immediate
- **Better tracking**: Store rejection reason + metadata
- **Configurable**: Tune cache/index durations per deployment
- **Observable**: Separate hot/cold metrics (future PR4)

### Future (PR3)
- **Immediate re-processing**: Get events from hot cache when valid
- **No 24h delay**: Maintainer announcements accepted in <1 second
- **Automatic recovery**: Hot cache for immediate, cold index for later

## Backward Compatibility

**No breaking changes:**
- Same rejection behavior (skip events in index)
- Same negentropy exclusion (union with purgatory IDs)
- Default config values match previous implicit behavior

**Migration:**
- Existing deployments continue working with defaults
- Optional: Tune durations via new config flags

## Testing

All tests passing:
- ✅ 9 rejected_index tests (hot cache, cold index, two-tier)
- ✅ 139 sync module tests (including updated integration tests)
- ✅ 247 total library tests

## Next Steps

**PR3: Add invalidation + immediate re-processing**
- Invalidate cold index when owner announcement accepted
- Get events from hot cache for re-processing
- Recursive call to process_event_static
- Integration tests for <1s maintainer acceptance

**PR4: Add cleanup + metrics**
- Hot cache cleanup task (every 60s)
- Cold index cleanup task (daily)
- Prometheus metrics for both tiers
- Monitor hot cache hits vs misses

## Configuration Examples

```bash
# Default (2 min hot cache, 7 day cold index)
ngit-grasp

# Longer hot cache for slow relays
ngit-grasp --rejected-hot-cache-duration-secs 300

# Shorter cold index for memory-constrained systems
ngit-grasp --rejected-cold-index-expiry-secs 86400

# Environment variables
export NGIT_REJECTED_HOT_CACHE_DURATION_SECS=180
export NGIT_REJECTED_COLD_INDEX_EXPIRY_SECS=259200
ngit-grasp
```

Part of: Maintainer chain discovery fix
See: work/SOLUTION-SUMMARY-V2.md for full design
Previous: PR1 (rejected_index.rs implementation)
Next: PR3 (invalidation + re-processing)

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