feat: agent tools as required tools with tenant tool gate

Agent definitions gain tools (websearch/imagegen/playwright,
unknown tool is a 400), stored as agent_tools JSON and returned
by GET /tasks. This is deliberately required-tools, not an
allowlist: create_thread has no tool allowlist and tenant homes
are per-project shared, so blocking other tools cannot be
claimed. Instead the worker fails fast before any model call
when a declared tool has no [mcp_servers.<tool>] section in
the tenant codex-home config.toml (checked after _tenant_dirs
seeding, so a section seeded earlier by another worker counts
as present). The gate runs only for tenant-isolated Docker
workers; local/cloud workers skip it, documented, not verified.

Tests: test_agent_tools.py (8 tests).

cloud/db.py

@@ -883,6 +883,7 @@ POSTGRES_SCHEMA = (
     "agent_name TEXT, "
     "agent_version TEXT, "
     "agent_instructions TEXT, "
+    "agent_tools TEXT, "
     "UNIQUE(project_id, idempotency_key))",
     "CREATE INDEX IF NOT EXISTS idx_tasks_status ON tasks(status)",
     # 附件包与任务附件：与 server._ensure_uploads_tables 同款，