Add compacted_at_commit field and git commit capture during compaction

- Add compacted_at_commit field to Issue type (bd-405)
- Add database schema and migration for new field
- Create GetCurrentCommitHash() helper function
- Update ApplyCompaction to store git commit hash (bd-395)
- Update compaction calls to capture current commit
- Update tests to verify commit hash storage
- All tests passing

Amp-Thread-ID: https://ampcode.com/threads/T-5518cccb-7fc9-4dcd-ba5a-e22cd10e45d7
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/schema.go

@@ -20,6 +20,7 @@ CREATE TABLE IF NOT EXISTS issues (
     external_ref TEXT,
     compaction_level INTEGER DEFAULT 0,
     compacted_at DATETIME,
+    compacted_at_commit TEXT,
     original_size INTEGER,
     CHECK ((status = 'closed') = (closed_at IS NOT NULL))
 );