Fix bd-v0y: Remove mtime fast-path in hasJSONLChanged

Git doesn't preserve mtime on checkout, causing false negatives where
hasJSONLChanged() incorrectly returns false after git pull updates JSONL.
This caused bd sync to overwrite pulled JSONL instead of importing it,
resurrecting deleted issues.

Solution: Always compute content hash for comparison (Option 1).
Performance impact is minimal (~10-50ms for sync operations).

Changes:
- cmd/bd/integrity.go: Remove mtime fast-path, always compute hash
- cmd/bd/sync.go: Remove mtime storage after import
- cmd/bd/import.go: Remove mtime storage after import
- cmd/bd/daemon_sync.go: Remove mtime storage and update comments
- cmd/bd/daemon_sync_test.go: Remove mtime assertions from tests

All tests pass. Existing test 'mtime changed but content same - git
operation scenario' verifies the fix works correctly.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/daemon_sync_test.go

@@ -476,24 +476,7 @@ func TestUpdateExportMetadataMultiRepo(t *testing.T) {
 		t.Error("expected global last_import_hash to not be set when using per-repo keys")
 	}
 
-	// Verify mtime metadata was also set per-repo (bd-web8: keys are sanitized)
-	mtime1Key := "last_import_mtime:" + sanitizeMetadataKey(jsonlPath1)
-	mtime1, err := store.GetMetadata(ctx, mtime1Key)
-	if err != nil {
-		t.Fatalf("failed to get %s: %v", mtime1Key, err)
-	}
-	if mtime1 == "" {
-		t.Errorf("expected %s to be set", mtime1Key)
-	}
-
-	mtime2Key := "last_import_mtime:" + sanitizeMetadataKey(jsonlPath2)
-	mtime2, err := store.GetMetadata(ctx, mtime2Key)
-	if err != nil {
-		t.Fatalf("failed to get %s: %v", mtime2Key, err)
-	}
-	if mtime2 == "" {
-		t.Errorf("expected %s to be set", mtime2Key)
-	}
+	// Note: last_import_mtime removed in bd-v0y fix (git doesn't preserve mtime)
 }
 
 // TestExportWithMultiRepoConfigUpdatesAllMetadata verifies that export with multi-repo
@@ -604,14 +587,7 @@ func TestExportWithMultiRepoConfigUpdatesAllMetadata(t *testing.T) {
 		t.Errorf("expected %s to be set after export", primaryTimeKey)
 	}
 
-	primaryMtimeKey := "last_import_mtime:" + sanitizeMetadataKey(primaryDir)
-	primaryMtime, err := store.GetMetadata(ctx, primaryMtimeKey)
-	if err != nil {
-		t.Fatalf("failed to get %s: %v", primaryMtimeKey, err)
-	}
-	if primaryMtime == "" {
-		t.Errorf("expected %s to be set after export", primaryMtimeKey)
-	}
+	// Note: last_import_mtime removed in bd-v0y fix (git doesn't preserve mtime)
 
 	// Verify metadata for additional repo (bd-web8: keys are sanitized)
 	additionalHashKey := "last_import_hash:" + sanitizeMetadataKey(additionalDir)
@@ -632,14 +608,7 @@ func TestExportWithMultiRepoConfigUpdatesAllMetadata(t *testing.T) {
 		t.Errorf("expected %s to be set after export", additionalTimeKey)
 	}
 
-	additionalMtimeKey := "last_import_mtime:" + sanitizeMetadataKey(additionalDir)
-	additionalMtime, err := store.GetMetadata(ctx, additionalMtimeKey)
-	if err != nil {
-		t.Fatalf("failed to get %s: %v", additionalMtimeKey, err)
-	}
-	if additionalMtime == "" {
-		t.Errorf("expected %s to be set after export", additionalMtimeKey)
-	}
+	// Note: last_import_mtime removed in bd-v0y fix (git doesn't preserve mtime)
 
 	// Note: In this test both JSONL files have the same content (all issues),
 	// so hashes will be identical. In real multi-repo mode, ExportToMultiRepo