feat: add NGIT_LOG_LEVEL configuration option

Add proper log level configuration following standard approach:
- CLI flag: --log-level <level>
- Environment variable: NGIT_LOG_LEVEL
- Default: info
- Supports simple levels (error, warn, info, debug, trace)
- Supports filter expressions (e.g., ngit_grasp=debug,actix_web=info)

Configuration is now consistent across all four sources:
1. src/config.rs - Config struct with log_level field
2. docs/reference/configuration.md - Full documentation
3. nix/module.nix - NixOS module with logLevel option
4. .env.example - Example configuration file

This replaces the previous RUST_LOG approach with proper integration
into the ngit-grasp configuration system, enabling trace logging from
CLI, environment variables, or NixOS configuration.

1 files changed, 5 insertions, 2 deletions

diff --git a/.env.example b/.env.example
index e152b89..01854f4 100644
--- a/.env.example
+++ b/.env.example
@@ -101,9 +101,12 @@
 # LOGGING
 # ============================================================================
 
-# Rust log level (not a ngit-grasp config, but useful for debugging)
+# Log level for application logging
+# CLI: --log-level <level>
+# Default: info
 # Options: error, warn, info, debug, trace
-# RUST_LOG=info
+# Can also use filter expressions: ngit_grasp=debug,actix_web=info
+# NGIT_LOG_LEVEL=info
 
 # ============================================================================
 # PROACTIVE SYNC (GRASP-02)