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/protocol.go

@@ -16,6 +16,7 @@ const (
 	OpList            = "list"
 	OpShow            = "show"
 	OpReady           = "ready"
+	OpStale           = "stale"
 	OpStats           = "stats"
 	OpDepAdd          = "dep_add"
 	OpDepRemove       = "dep_remove"
@@ -118,6 +119,13 @@ type ReadyArgs struct {
 	SortPolicy string `json:"sort_policy,omitempty"`
 }
 
+// StaleArgs represents arguments for the stale command
+type StaleArgs struct {
+	Days   int    `json:"days,omitempty"`
+	Status string `json:"status,omitempty"`
+	Limit  int    `json:"limit,omitempty"`
+}
+
 // DepAddArgs represents arguments for adding a dependency
 type DepAddArgs struct {
 	FromID  string `json:"from_id"`