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/client.go

@@ -265,6 +265,11 @@ func (c *Client) Ready(args *ReadyArgs) (*Response, error) {
 	return c.Execute(OpReady, args)
 }
 
+// Stale gets stale issues via the daemon
+func (c *Client) Stale(args *StaleArgs) (*Response, error) {
+	return c.Execute(OpStale, args)
+}
+
 // Stats gets statistics via the daemon
 func (c *Client) Stats() (*Response, error) {
 	return c.Execute(OpStats, nil)