refactor: make wrapper daemon ownership explicit

[ejgallego]

This PR makes wrapper daemon ownership explicit so daemon lifetime is tied to a deliberate lean-beam ensure --hold session instead of inferred from per-request leases. This removes the lease, heartbeat, revocation, and retirement protocol while giving wrapper commands deterministic attachment and shutdown behavior.

• Only lean-beam ensure --hold starts and owns a wrapper daemon; ordinary commands attach to that session and report a direct recovery command when no owner exists.
• The holder owns the daemon through an inherited stdin pipe. Holder exit closes the pipe and stops admission, cancels active requests, shuts down backend sessions, and exits the daemon.
• Explicit shutdown removes only the matching registry generation, which tells the holder to close its ownership pipe; typed admission closure ensures backend teardown happens once under shutdown races.
• Registry generation, endpoint, root, and PID-domain validation remain fail-closed, while MCP keeps its independent stdio-owned lifetime.