feat(M5-004,M5-005,M5-006,M5-007): session-conversation binding, session:info broadcast, agent creation from TUI, and session metrics

- M5-004: Add `session_id` column to conversations table (nullable text) with Drizzle migration 0006_swift_shen.sql; bind sessionId to conversation on session creation; reuse existing sessionId when resuming a conversation
- M5-005: Emit session:info with agentName on model/agent switch via broadcastSessionInfo(); set:thinking and setModelOverride now broadcast updated session:info to all clients on a conversation
- M5-006: /agent new <name> gateway command creates an agent config via brain.agents.create() with defaults (provider/model from env vars) and returns agentId+agentName in command:result data
- M5-007: Track per-session metrics (input/output/cache tokens, modelSwitches, messageCount, lastActivityAt) accumulated across turns; exposed via GET /api/sessions/:id metrics field

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

packages/db/drizzle/0006_swift_shen.sql

@@ -0,0 +1 @@
+ALTER TABLE "conversations" ADD COLUMN "session_id" text;

packages/db/drizzle/meta/_journal.json

@@ -43,6 +43,13 @@
       "when": 1774225763410,
       "tag": "0005_minor_champions",
       "breakpoints": true
+    },
+    {
+      "idx": 6,
+      "version": "7",
+      "when": 1774227064500,
+      "tag": "0006_swift_shen",
+      "breakpoints": true
     }
   ]
 }

packages/db/src/schema.ts

@@ -319,6 +319,8 @@ export const conversations = pgTable(
       .references(() => users.id, { onDelete: 'cascade' }),
     projectId: uuid('project_id').references(() => projects.id, { onDelete: 'set null' }),
     agentId: uuid('agent_id').references(() => agents.id, { onDelete: 'set null' }),
+    /** M5-004: Agent session ID bound to this conversation. Nullable — set when a session is created. */
+    sessionId: text('session_id'),
     archived: boolean('archived').notNull().default(false),
     createdAt: timestamp('created_at', { withTimezone: true }).notNull().defaultNow(),
     updatedAt: timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),