build(deps): update dependencies and implement new acp.Agent methods

- Bump fantasy v0.21.0 -> v0.23.0, mcp-go v0.49.0 -> v0.51.0,
  acp-go-sdk v0.12.0 -> v0.12.2, chroma v2.23.1 -> v2.24.1,
  fsnotify v1.9.0 -> v1.10.1, ultraviolet, AWS SDK, Google API
- Implement CloseSession and ResumeSession on acpserver.Agent to
  satisfy the expanded acp.Agent interface in acp-go-sdk v0.12.2
- Add sessionRegistry.remove helper to support session close

internal/acpserver/agent.go

@@ -185,6 +185,26 @@ func (a *Agent) ListSessions(_ context.Context, _ acp.ListSessionsRequest) (acp.
 	}, nil
 }
 
+// CloseSession cancels any ongoing work for the session and frees its resources.
+func (a *Agent) CloseSession(_ context.Context, params acp.CloseSessionRequest) (acp.CloseSessionResponse, error) {
+	sessionID := string(params.SessionId)
+	sess, ok := a.registry.get(sessionID)
+	if !ok {
+		return acp.CloseSessionResponse{}, nil
+	}
+
+	log.Debug("acp: close session", "session", sessionID)
+	sess.cancelPrompt()
+	a.registry.remove(sessionID)
+	return acp.CloseSessionResponse{}, nil
+}
+
+// ResumeSession is not supported — Kit doesn't persist sessions across
+// restarts in ACP mode. Clients should use NewSession instead.
+func (a *Agent) ResumeSession(_ context.Context, _ acp.ResumeSessionRequest) (acp.ResumeSessionResponse, error) {
+	return acp.ResumeSessionResponse{}, fmt.Errorf("resume session not supported")
+}
+
 // SetSessionConfigOption handles session configuration changes. Currently
 // supports the "model" config option to change the active model for a session.
 func (a *Agent) SetSessionConfigOption(ctx context.Context, params acp.SetSessionConfigOptionRequest) (acp.SetSessionConfigOptionResponse, error) {

internal/acpserver/session.go

@@ -232,6 +232,20 @@ func (r *sessionRegistry) closeAll() {
 	}
 }
 
+// remove closes and removes a single session by ID.
+func (r *sessionRegistry) remove(sessionID string) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	sess, ok := r.sessions[sessionID]
+	if !ok {
+		return
+	}
+	if sess.kit != nil {
+		_ = sess.kit.Close()
+	}
+	delete(r.sessions, sessionID)
+}
+
 // cancelPrompt cancels the current prompt for a session, if any.
 func (s *acpSession) cancelPrompt() {
 	s.cancelMu.Lock()