Fix bd-160: Implement JSONL integrity validation and prevent export deduplication data loss

## Problem
Export deduplication feature broke when JSONL and export_hashes diverged
(e.g., after git pull/reset). This caused exports to skip issues that
weren't actually in the file, leading to silent data loss.

## Solution
1. JSONL integrity validation before every export
   - Store JSONL file hash after export
   - Validate hash before export, clear export_hashes if mismatch
   - Automatically recovers from git operations changing JSONL

2. Clear export_hashes on all imports
   - Prevents stale hashes from causing future export failures
   - Import operations invalidate export_hashes state

3. Add Storage interface methods:
   - GetJSONLFileHash/SetJSONLFileHash for integrity tracking
   - ClearAllExportHashes for recovery

## Tests Added
- TestJSONLIntegrityValidation: Unit tests for validation logic
- TestImportClearsExportHashes: Verifies imports clear hashes
- TestExportIntegrityAfterJSONLTruncation: Simulates git reset (would have caught bd-160)
- TestExportIntegrityAfterJSONLDeletion: Tests recovery from file deletion
- TestMultipleExportsStayConsistent: Tests repeated export integrity

## Follow-up
Created bd-179 epic for remaining integration test gaps (multi-repo sync,
daemon auto-sync, corruption recovery tests).

Closes bd-160

internal/storage/sqlite/hash.go

@@ -50,3 +50,36 @@ func (s *SQLiteStorage) ClearAllExportHashes(ctx context.Context) error {
 	}
 	return nil
 }
+
+// GetJSONLFileHash retrieves the stored hash of the JSONL file.
+// Returns empty string if no hash is stored (bd-160).
+func (s *SQLiteStorage) GetJSONLFileHash(ctx context.Context) (string, error) {
+	var hash string
+	err := s.db.QueryRowContext(ctx, `
+		SELECT value FROM metadata WHERE key = 'jsonl_file_hash'
+	`).Scan(&hash)
+	
+	if err == sql.ErrNoRows {
+		return "", nil // No hash stored yet
+	}
+	if err != nil {
+		return "", fmt.Errorf("failed to get jsonl_file_hash: %w", err)
+	}
+	
+	return hash, nil
+}
+
+// SetJSONLFileHash stores the hash of the JSONL file after export (bd-160).
+func (s *SQLiteStorage) SetJSONLFileHash(ctx context.Context, fileHash string) error {
+	_, err := s.db.ExecContext(ctx, `
+		INSERT INTO metadata (key, value)
+		VALUES ('jsonl_file_hash', ?)
+		ON CONFLICT(key) DO UPDATE SET value = excluded.value
+	`, fileHash)
+	
+	if err != nil {
+		return fmt.Errorf("failed to set jsonl_file_hash: %w", err)
+	}
+	
+	return nil
+}