## 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
c34b93fa1a