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/types/types.go

@@ -25,6 +25,7 @@ type Issue struct {
 	ExternalRef        *string        `json:"external_ref,omitempty"` // e.g., "gh-9", "jira-ABC"
 	CompactionLevel    int            `json:"compaction_level,omitempty"`
 	CompactedAt        *time.Time     `json:"compacted_at,omitempty"`
+	CompactedAtCommit  *string        `json:"compacted_at_commit,omitempty"` // Git commit hash when compacted
 	OriginalSize       int            `json:"original_size,omitempty"`
 	Labels             []string       `json:"labels,omitempty"`       // Populated only for export/import
 	Dependencies       []*Dependency  `json:"dependencies,omitempty"` // Populated only for export/import