Replace mtime-based staleness detection with content-based (SHA256 hash) to prevent
git operations from resurrecting deleted issues.
**Problem:**
Auto-import used file modification time to detect if JSONL was "newer" than database.
Git operations (checkout, merge, pull) restore old files with recent mtimes, causing
auto-import to load stale data over current database state, resurrecting deleted issues.
**Solution:**
- Added computeJSONLHash() to compute SHA256 of JSONL content
- Added hasJSONLChanged() with two-tier check:
1. Fast-path: Check mtime first (99% of checks are instant)
2. Slow-path: Compute hash only if mtime changed (catches git operations)
- Store metadata: last_import_hash, last_import_mtime, last_import_time
- Updated auto-import in daemon_sync.go to use content-based check
- Updated validatePreExport to use content-based check (bd-xwo)
- Graceful degradation: metadata failures are non-fatal warnings
**Changes:**
- cmd/bd/integrity.go: Add computeJSONLHash(), hasJSONLChanged()
- cmd/bd/integrity_test.go: Add comprehensive tests for new functions
- cmd/bd/import.go: Update metadata after import
- cmd/bd/sync.go: Use hasJSONLChanged() instead of isJSONLNewer()
- cmd/bd/daemon_sync.go: Use hasJSONLChanged() in auto-import
**Testing:**
- Unit tests pass (TestHasJSONLChanged with 7 scenarios)
- Integration test passes (test_bd_khnb_fix.sh)
- Verified git resurrection scenario prevented
Fixes: bd-khnb
Related: bd-3bg, bd-xwo, bd-39o, bd-56p, bd-m8t, bd-rfj, bd-t5o
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
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