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/storage.go

@@ -59,6 +59,11 @@ type Storage interface {
 	// Export hash tracking (for timestamp-only dedup, bd-164)
 	GetExportHash(ctx context.Context, issueID string) (string, error)
 	SetExportHash(ctx context.Context, issueID, contentHash string) error
+	ClearAllExportHashes(ctx context.Context) error
+	
+	// JSONL file integrity (bd-160)
+	GetJSONLFileHash(ctx context.Context) (string, error)
+	SetJSONLFileHash(ctx context.Context, fileHash string) error
 
 	// Config
 	SetConfig(ctx context.Context, key, value string) error