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 (re-adoption). HOST_CONTROLLER is the separate
  domain for the controller's own lifecycle endpoints, keyed by a key the
  orchestrator never holds (its lifecycle role is `host`, deliberately
  outside the control-plane ROLES). LaunchBrokerProvisioning is the
  fail-closed seam, mirroring ControlPlaneProvisioning.
- orchestrator_auth.py: ROLE_HOST, outside ROLES so the orchestrator's
  control-plane key can neither mint nor accept it.
- paths.py: key-file + env-var constants for both domains.
- host_server.py / __main__.py: broker_secret() now resolves the durable
  LAUNCH_BROKER key — env-injected for a containerized launcher, else the
  host key file for a host-side dev-harness process — so `--broker http`
  and the host controller "just work" on one host without exporting a
  secret, and stay fail-closed when the root is unwritable.

Tested: domain boundary (host role unmintable by the control plane,
cross-domain tokens don't verify), distinct keys/env vars per domain,
provisioning fail-closed, and broker_secret env/file resolution +
durability. Full unit suite green; pyright clean; pylint 9.90.

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"]