feat(agents): Tier 2 — AGENTS.md + per-session picker

Six builtin defaults (Code Reviewer, Debugger, Refactorer, Architect,
Security Auditor, Prompt Builder) with no model field so session.model
wins. Project root AGENTS.md parsed on demand with mtime cache; when
present, only its agents are shown. sessions.agent_id resolves per turn
into effective system prompt, temperature, and a tool whitelist applied
in inference. AgentPicker mounts in the ChatInput toolbar; SettingsDrawer
agent surface deferred to Batch 7.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/server/src/schema.sql

@@ -153,3 +153,8 @@ BEGIN
       CHECK (status IN ('open', 'archived'));
   END IF;
 END $$;
+
+-- v1.x-batch9: per-session agent reference. Agent definitions are not stored in
+-- the DB; they live in builtins (services/agents.ts) and a per-project AGENTS.md.
+-- agent_id is the slugified agent name. NULL means "use BooCode defaults".
+ALTER TABLE sessions ADD COLUMN IF NOT EXISTS agent_id TEXT;