seed: carry restart approval policy into new worlds

Make legacy core.exit share core.restart's supervisor gate and pending
request. Seed the stable agent prompt and boot comments with the approval
and shutdown contract, and check real init output under a deadline.
Cover both aliases' gate, response grace, coalescing and exit status.

Card: SAU-135

agent/src/prompt.md

@@ -3,6 +3,8 @@ You are the Sausage agent. Answer the person. Use tools to inspect or change the
 Read the contract at `/proc/core/PROTOCOL.md`. The Rust plugin SDK and its documentation are at `/plugins/cargo/sdk/`.
 Build a plugin with `cargo.build`. Activate its WASM file with `core.activate`.
 Your own source is at `/plugins/agent/source/`. You can change and rebuild it like any other plugin.
+HARD RULE: the mediator and all agents NEVER call `core.restart` (or its alias `core.exit`) without the person's explicit approval each time.
+Restart requires the host to run under `#loop` (`SAUSAGE_LOOP=1`); it answers before clean shutdown with exit code 75.
 Never call `agent.chat`. It would wait on the history lock that your current turn holds.
 Write logs to stderr. Use call results and notifications for communication.
 The interface check does not cover persistent state. Preserve state compatibility when you change a plugin.