feat(orchestrator): durable launch-broker secret via TrustDomain (#468)

Chunk 2 of the host-control-server stack: close the PRD's **durable
secret** gap and replace chunk 1's BOT_BOTTLE_BROKER_SECRET stopgap.

- trust_domain.py: two new domains. LAUNCH_BROKER holds the durable
  HS256 key both the orchestrator (signer) and the host control server
  (verifier) share for the broker's launch JWT — a host-canonical key
  file minted 0600 on first use, so a restarted orchestrator re-verifies
  against the same key. HOST_CONTROLLER is the separate domain for the
  controller's own lifecycle endpoints, keyed by a key the orchestrator
  never holds (its role is `host`, deliberately outside control-plane
  ROLES). LaunchBrokerProvisioning is the fail-closed seam.
- orchestrator_auth.py: ROLE_HOST, outside ROLES.
- paths.py: key-file + env-var constants for both domains.

Key resolution is split by owner (addresses codex review on #497):
broker_secret(allow_host_file=...) — the host controller / dev-harness
(True) may mint/read the durable host key file it owns; the GUEST
orchestrator (--broker http, default False) must be *injected* the key
and fails closed if it isn't. A guest that fell back to the host file
would mint a process-local key unrelated to the host controller's, so
startup would succeed but every launch would 401 — this prevents that
silent divergence.

Tested: domain boundary + separation, provisioning fail-closed, and
broker_secret env-only (guest) vs host-file (host) resolution. pyright
clean; pylint 9.86.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/orchestrator_auth.py

@@ -36,6 +36,13 @@ ROLE_GATEWAY = "gateway"
 ROLE_CLI = "cli"
 ROLES: frozenset[str] = frozenset({ROLE_GATEWAY, ROLE_CLI})
 
+# The host controller's own lifecycle role (#468). Deliberately OUTSIDE `ROLES`:
+# it belongs to a separate trust domain (`HOST_CONTROLLER`) signed by a key the
+# orchestrator never holds, so the orchestrator's control-plane key can neither
+# mint nor accept it — the orchestrator must not be able to forge the credential
+# used to start and stop it.
+ROLE_HOST = "host"
+
 _ALG = "HS256"
 
 
@@ -103,4 +110,4 @@ def verify(token: str, secret: str, *, roles: frozenset[str] = ROLES) -> str | N
     return role if isinstance(role, str) and role in roles else None
 
 
-__all__ = ["ROLE_GATEWAY", "ROLE_CLI", "ROLES", "mint", "verify"]
+__all__ = ["ROLE_GATEWAY", "ROLE_CLI", "ROLE_HOST", "ROLES", "mint", "verify"]