WIP: bd-164 timestamp-only export deduplication (~80% complete)

Implemented content hash-based deduplication to skip exports when only
timestamps changed. Core logic complete, needs export_hashes table wiring.

Completed:
- Added computeIssueContentHash() excluding timestamps
- Created shouldSkipExport() logic
- Updated export loop to skip timestamp-only changes
- Added hash.go with content hashing
- Extended Storage interface

Remaining:
- Complete export_hashes table migration
- Add SetExportHash/GetExportHash to interface
- Revert content_hash from dirty_issues approach
- Wire up hash persistence in export
- Testing

See bd-164 notes for details.

Amp-Thread-ID: https://ampcode.com/threads/T-d70657d1-4433-4f7e-b10a-3fccf8bf17fb
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/storage.go

@@ -52,6 +52,7 @@ type Storage interface {
 
 	// Dirty tracking (for incremental JSONL export)
 	GetDirtyIssues(ctx context.Context) ([]string, error)
+	GetDirtyIssueHash(ctx context.Context, issueID string) (string, error) // For timestamp-only dedup (bd-164)
 	ClearDirtyIssues(ctx context.Context) error // WARNING: Race condition (bd-52), use ClearDirtyIssuesByID
 	ClearDirtyIssuesByID(ctx context.Context, issueIDs []string) error