FEAT-011: daemon should refuse to boot under production-leaked AGENTTOWER_TEST_FORCE_HOST_PEER

[brettheap]

Summary

The AGENTTOWER_TEST_FORCE_HOST_PEER=1 test seam makes the FEAT-011 host-vs-container peer check (_peer_is_host_process in socket_api/methods.py) return True for any caller — a process-wide bypass of the FR-042 host-only gate. It exists because WSL2 / Docker-in-Docker / sandboxed CI runners false-positive the /proc container probe, so the FEAT-011 integration suite cannot run without it.

A prior remediation (PR #19, commit 0aaeb1e) made the bypass auditable: the daemon emits a host_peer_check_bypassed warn-level lifecycle event at startup when the seam is set. That is a mitigation, not a fix — the daemon still boots and serves with the gate disabled.

Desired hardening

The daemon should refuse to boot when AGENTTOWER_TEST_FORCE_HOST_PEER is set in a production-shaped environment — mirroring the existing _guard_production_test_seam_unset() pattern in cli.py (which refuses a lone AGENTTOWER_TEST_PROC_ROOT).

The constraint that deferred it

tests/integration/_daemon_helpers.py currently sets AGENTTOWER_TEST_FORCE_HOST_PEER=1 as the only AGENTTOWER_TEST_* var in the test daemon's environment. A naive "refuse if set without a companion AGENTTOWER_TEST_* var" guard would therefore break the entire FEAT-011 integration suite. The fix needs a coordinated change:

1. Add a daemon-side guard (daemon.py startup) that refuses to boot when AGENTTOWER_TEST_FORCE_HOST_PEER is set without a recognized companion test seam.
2. Update tests/integration/_daemon_helpers.py to set a companion AGENTTOWER_TEST_* marker alongside it, so the integration suite still boots.

Acceptance criteria

• A production agenttowerd invocation with AGENTTOWER_TEST_FORCE_HOST_PEER=1 set (and no companion test seam) exits non-zero with a clear error, rather than booting with the gate bypassed.
• The FEAT-011 integration suite (tests/integration/test_story*.py) still passes — the test daemon boots because _daemon_helpers.py sets the companion marker.
• The host_peer_check_bypassed warn-event remains for the legitimate test-harness path.

Found by the FEAT-011 orchestrated code review. Tracked deferred from PR #19.

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