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/orchestrator_auth.py

@@ -59,12 +59,19 @@ _HEADER_SEGMENT = _b64url_encode(
 )
 
 
-def mint(role: str, secret: str) -> str:
+def mint(role: str, secret: str, *, roles: frozenset[str] = ROLES) -> str:
     """A compact HS256 token asserting `role`, signed with `secret`.
 
-    Raises ValueError for an unknown role (mint only what the control plane will
+    `roles` is the role set the caller's *trust domain* recognises (default: the
+    orchestrator control plane's `{gateway, cli}`). A domain names its own set so
+    each credential boundary mints only its own roles — a separate boundary
+    (e.g. a host controller) instantiates a distinct domain with a distinct key
+    and role set rather than adding a role here, so its key cannot forge the
+    other domain's tokens (see `trust_domain.py`, issues #476/#468).
+
+    Raises ValueError for a role outside `roles` (mint only what that domain will
     accept) or an empty signing key (an unsigned credential is never valid)."""
-    if role not in ROLES:
+    if role not in roles:
         raise ValueError(f"unknown control-plane role {role!r}")
     if not secret:
         raise ValueError("cannot mint a control-plane token without a signing key")
@@ -73,10 +80,11 @@ def mint(role: str, secret: str) -> str:
     return f"{signing_input}.{_sign(secret, signing_input)}"
 
 
-def verify(token: str, secret: str) -> str | None:
+def verify(token: str, secret: str, *, roles: frozenset[str] = ROLES) -> str | None:
     """The role a valid `token` carries, or None if it is malformed, wrongly
-    signed, or names an unknown role. Constant-time signature check; rejects any
-    header whose alg isn't HS256 (no alg-confusion / `none`)."""
+    signed, or names a role outside `roles` (the verifying trust domain's set —
+    default `{gateway, cli}`). Constant-time signature check; rejects any header
+    whose alg isn't HS256 (no alg-confusion / `none`)."""
     if not token or not secret:
         return None
     parts = token.split(".")
@@ -94,7 +102,7 @@ def verify(token: str, secret: str) -> str | None:
     if not isinstance(header, dict) or header.get("alg") != _ALG:
         return None
     role = payload.get("role") if isinstance(payload, dict) else None
-    return role if isinstance(role, str) and role in ROLES else None
+    return role if isinstance(role, str) and role in roles else None
 
 
 __all__ = ["ROLE_GATEWAY", "ROLE_CLI", "ROLES", "mint", "verify"]