refactor(orchestrator): uniform control-plane auth provisioning per trust domain

Hoist control-plane auth provisioning out of the per-backend launchers into
one shared contract, parameterized per trust domain (#476). Every blocking
finding in PR #471 was the same integration-bug class: each launcher
re-derived, by hand, how to generate the signing key, scope it to the
orchestrator, mint the gateway JWT, and keep the host key canonical.

Introduces `trust_domain.py`:

  * `TrustDomain` — one credential boundary (host-canonical key file + role
    set + env vars). `mint`/`verify` are scoped to the domain's roles, so a
    future host-controller domain (#468) uses its own key/verifier/roles
    rather than a `host` role on the control plane's frozenset (which the
    orchestrator key could then forge).
  * `ControlPlaneProvisioning` — the single seam answering the four
    invariants: host-canonical key, split key-vs-token credential, CLI token
    valid across co-running backends, and fail-closed (no open mode) for any
    co-located topology.
  * `Topology` — the backend declares what it is; the default is co-located +
    fail-closed, so a backend need not redeclare it.

The `Orchestrator` ABC gets `control_plane_key()` (fail-closed) and routes
`mint_gateway_token()` through the contract; docker/macOS/firecracker
orchestrators, the server (verify), and the host CLI client (mint cli) all go
through the domain instead of reading the host key directly. `orchestrator_auth`
gains an optional `roles=` arg (default unchanged) so a domain scopes its own
role set; `paths.host_signing_key(filename)` generalizes host_orchestrator_token.

Adds unit coverage for the domain boundary + provisioning invariants and a PRD
capturing the durable rationale. No change to the auth primitive's HMAC, the
plane split, or the server's documented open-mode fallback.

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

Closes #476

bot_bottle/backend/docker/orchestrator.py

@@ -19,7 +19,6 @@ from .util import run_docker
 from ...paths import (
     ORCHESTRATOR_TOKEN_ENV,
     bot_bottle_root,
-    host_orchestrator_token,
 )
 from ...gateway import GatewayError
 from ...orchestrator.lifecycle import (
@@ -171,7 +170,9 @@ class DockerOrchestrator(Orchestrator):
         fixed-name container first)."""
         self._ensure_control_network()
         run_docker(["docker", "rm", "--force", self.name])
-        _signing_key = host_orchestrator_token()
+        # The signing key comes through the shared provisioning contract (#476),
+        # which fail-closes rather than yield an empty key that would run OPEN.
+        _signing_key = self.control_plane_key()
         proc = run_docker([
             "docker", "run", "--detach",
             "--name", self.name,