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/rpc/server_routing_validation_diagnostics.go

@@ -172,6 +172,8 @@ func (s *Server) handleRequest(req *Request) Response {
 		resp = s.handleResolveID(req)
 	case OpReady:
 		resp = s.handleReady(req)
+	case OpStale:
+		resp = s.handleStale(req)
 	case OpStats:
 		resp = s.handleStats(req)
 	case OpDepAdd: