feat(M5-004,M5-005,M5-006,M5-007): session-conversation binding, session:info broadcast, agent creation from TUI, and session metrics (#321)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

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(),