Implement bd stale command (bd-c01f, closes #184)

- Add bd stale command to find abandoned/forgotten issues
- Support --days (default 30), --status, --limit, --json flags
- Implement GetStaleIssues in SQLite and Memory storage
- Add full RPC/daemon support
- Comprehensive test suite (6 tests, all passing)
- Update AGENTS.md documentation

Resolves GitHub issue #184

Amp-Thread-ID: https://ampcode.com/threads/T-f021ddb8-54e3-41bf-ba7a-071749663c1d
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/storage.go

@@ -38,6 +38,7 @@ type Storage interface {
 	GetReadyWork(ctx context.Context, filter types.WorkFilter) ([]*types.Issue, error)
 	GetBlockedIssues(ctx context.Context) ([]*types.BlockedIssue, error)
 	GetEpicsEligibleForClosure(ctx context.Context) ([]*types.EpicStatus, error)
+	GetStaleIssues(ctx context.Context, filter types.StaleFilter) ([]*types.Issue, error)
 
 	// Events
 	AddComment(ctx context.Context, issueID, actor, comment string) error