feat(M4-007,M4-008,M4-012): wire routing engine into ChatGateway, add /model override and transparency

M4-012: In ChatGateway.handleMessage(), call routingEngine.resolve() before
creating a new agent session when no explicit provider/model is supplied.
The routing decision's provider/model are passed to AgentService.createSession().

M4-007: Add sticky per-session /model override via ChatGateway.setModelOverride().
When active, the routing engine is bypassed entirely. CommandExecutorService
handleModel() is updated to set/clear the override. /model clear resets to
automatic routing. /model with no args shows the current override or usage hint.

M4-008: Include routingDecision in the session:info socket event.
Add RoutingDecisionInfo to SessionInfoPayload in @mosaic/types.
useSocket() tracks the routing decision in state and the BottomBar TUI component
displays a "Routed: <model> (<reason>)" line when a routing decision is present.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/types/src/chat/events.ts

@@ -74,6 +74,14 @@ export interface ChatMessagePayload {
   agentId?: string;
 }
 
+/** Routing decision summary included in session:info for transparency */
+export interface RoutingDecisionInfo {
+  model: string;
+  provider: string;
+  ruleName: string;
+  reason: string;
+}
+
 /** Session info pushed when session is created or model changes */
 export interface SessionInfoPayload {
   conversationId: string;
@@ -81,6 +89,8 @@ export interface SessionInfoPayload {
   modelId: string;
   thinkingLevel: string;
   availableThinkingLevels: string[];
+  /** Present when automatic routing determined the model for this session */
+  routingDecision?: RoutingDecisionInfo;
 }
 
 /** Client request to change thinking level */

packages/types/src/chat/index.ts

@@ -9,6 +9,7 @@ export type {
   ToolEndPayload,
   SessionUsagePayload,
   SessionInfoPayload,
+  RoutingDecisionInfo,
   SetThinkingPayload,
   ErrorPayload,
   ChatMessagePayload,