Fix bd-khnb: Prevent auto-import from resurrecting deleted issues

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>

cmd/bd/daemon_sync.go

@@ -385,26 +385,13 @@ func createAutoImportFunc(ctx context.Context, store storage.Storage, log daemon
 			log.log("Removed stale lock (%s), proceeding", holder)
 		}
 
-		// Check JSONL modification time to avoid redundant imports
-		jsonlInfo, err := os.Stat(jsonlPath)
-		if err != nil {
-			log.log("Failed to stat JSONL: %v", err)
-			return
-		}
-
-		// Get database modification time
-		dbPath := filepath.Join(beadsDir, "beads.db")
-		dbInfo, err := os.Stat(dbPath)
-		if err != nil {
-			log.log("Failed to stat database: %v", err)
-			return
-		}
-
-		// Skip if JSONL is older than database (nothing new to import)
-		if !jsonlInfo.ModTime().After(dbInfo.ModTime()) {
-			log.log("Skipping import: JSONL not newer than database")
+		// Check JSONL content hash to avoid redundant imports
+		// Use content-based check (not mtime) to avoid git resurrection bug (bd-khnb)
+		if !hasJSONLChanged(importCtx, store, jsonlPath) {
+			log.log("Skipping import: JSONL content unchanged")
 			return
 		}
+		log.log("JSONL content changed, proceeding with import...")
 
 		// Pull from git (try sync branch first)
 		pulled, err := syncBranchPull(importCtx, store, log)