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/cli/src/tui/app.tsx

@@ -403,6 +403,7 @@ export function TuiApp({
         providerName={socket.providerName}
         thinkingLevel={socket.thinkingLevel}
         conversationId={socket.conversationId}
+        routingDecision={socket.routingDecision}
       />
     </Box>
   );

packages/cli/src/tui/components/bottom-bar.tsx

@@ -1,5 +1,6 @@
 import React from 'react';
 import { Box, Text } from 'ink';
+import type { RoutingDecisionInfo } from '@mosaic/types';
 import type { TokenUsage } from '../hooks/use-socket.js';
 import type { GitInfo } from '../hooks/use-git-info.js';
 
@@ -12,6 +13,8 @@ export interface BottomBarProps {
   providerName: string | null;
   thinkingLevel: string;
   conversationId: string | undefined;
+  /** Routing decision info for transparency display (M4-008) */
+  routingDecision?: RoutingDecisionInfo | null;
 }
 
 function formatTokens(n: number): string {
@@ -38,6 +41,7 @@ export function BottomBar({
   providerName,
   thinkingLevel,
   conversationId,
+  routingDecision,
 }: BottomBarProps) {
   const gatewayStatus = connected ? 'Connected' : connecting ? 'Connecting…' : 'Disconnected';
   const gatewayColor = connected ? 'green' : connecting ? 'yellow' : 'red';
@@ -120,6 +124,15 @@ export function BottomBar({
           </Text>
         </Box>
       </Box>
+
+      {/* Line 4: routing transparency (M4-008) — only shown when a routing decision is available */}
+      {routingDecision && (
+        <Box>
+          <Text dimColor>
+            Routed: {routingDecision.model} ({routingDecision.reason})
+          </Text>
+        </Box>
+      )}
     </Box>
   );
 }

packages/cli/src/tui/hooks/use-socket.ts

@@ -14,6 +14,7 @@ import type {
   CommandManifestPayload,
   SlashCommandResultPayload,
   SystemReloadPayload,
+  RoutingDecisionInfo,
 } from '@mosaic/types';
 import { commandRegistry } from '../commands/index.js';
 
@@ -66,6 +67,8 @@ export interface UseSocketReturn {
   providerName: string | null;
   thinkingLevel: string;
   availableThinkingLevels: string[];
+  /** Last routing decision received from the gateway (M4-008) */
+  routingDecision: RoutingDecisionInfo | null;
   sendMessage: (content: string) => void;
   addSystemMessage: (content: string) => void;
   setThinkingLevel: (level: string) => void;
@@ -109,6 +112,7 @@ export function useSocket(opts: UseSocketOptions): UseSocketReturn {
   const [providerName, setProviderName] = useState<string | null>(null);
   const [thinkingLevel, setThinkingLevelState] = useState<string>('off');
   const [availableThinkingLevels, setAvailableThinkingLevels] = useState<string[]>([]);
+  const [routingDecision, setRoutingDecision] = useState<RoutingDecisionInfo | null>(null);
   const [connectionError, setConnectionError] = useState<string | null>(null);
 
   const socketRef = useRef<TypedSocket | null>(null);
@@ -154,6 +158,10 @@ export function useSocket(opts: UseSocketOptions): UseSocketReturn {
       setModelName(data.modelId);
       setThinkingLevelState(data.thinkingLevel);
       setAvailableThinkingLevels(data.availableThinkingLevels);
+      // Update routing decision if provided (M4-008)
+      if (data.routingDecision) {
+        setRoutingDecision(data.routingDecision);
+      }
     });
 
     socket.on('agent:start', () => {
@@ -319,6 +327,7 @@ export function useSocket(opts: UseSocketOptions): UseSocketReturn {
     providerName,
     thinkingLevel,
     availableThinkingLevels,
+    routingDecision,
     sendMessage,
     addSystemMessage,
     setThinkingLevel,