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/types/types.go

@@ -289,6 +289,13 @@ type WorkFilter struct {
 	SortPolicy SortPolicy
 }
 
+// StaleFilter is used to filter stale issue queries
+type StaleFilter struct {
+	Days   int    // Issues not updated in this many days
+	Status string // Filter by status (open|in_progress|blocked), empty = all non-closed
+	Limit  int    // Maximum issues to return
+}
+
 // EpicStatus represents an epic with its completion status
 type EpicStatus struct {
 	Epic            *Issue `json:"epic"`