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/memory/memory.go

@@ -595,6 +595,24 @@ func (m *MemoryStorage) SetExportHash(ctx context.Context, issueID, hash string)
 	return nil
 }
 
+// ClearAllExportHashes clears all export hashes
+func (m *MemoryStorage) ClearAllExportHashes(ctx context.Context) error {
+	// Memory storage doesn't track export hashes, no-op
+	return nil
+}
+
+// GetJSONLFileHash gets the JSONL file hash
+func (m *MemoryStorage) GetJSONLFileHash(ctx context.Context) (string, error) {
+	// Memory storage doesn't track JSONL file hashes, return empty string
+	return "", nil
+}
+
+// SetJSONLFileHash sets the JSONL file hash
+func (m *MemoryStorage) SetJSONLFileHash(ctx context.Context, fileHash string) error {
+	// Memory storage doesn't track JSONL file hashes, no-op
+	return nil
+}
+
 // GetDependencyTree gets the dependency tree for an issue
 func (m *MemoryStorage) GetDependencyTree(ctx context.Context, issueID string, maxDepth int, showAllPaths bool, reverse bool) ([]*types.TreeNode, error) {
 	// Simplified implementation - just return direct dependencies