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/importer/importer.go

@@ -76,6 +76,14 @@ func ImportIssues(ctx context.Context, dbPath string, store storage.Storage, iss
 	if needCloseStore {
 		defer func() { _ = sqliteStore.Close() }()
 	}
+	
+	// Clear export_hashes before import to prevent staleness (bd-160)
+	// Import operations may add/update issues, so export_hashes entries become invalid
+	if !opts.DryRun {
+		if err := sqliteStore.ClearAllExportHashes(ctx); err != nil {
+			fmt.Fprintf(os.Stderr, "Warning: failed to clear export_hashes before import: %v\n", err)
+		}
+	}
 
 	// Check and handle prefix mismatches
 	if err := handlePrefixMismatch(ctx, sqliteStore, issues, opts, result); err != nil {