Implement edge-host-self-enrollment

Host Agent:
- One-time local operator account bootstrap (PBKDF2-HMAC-SHA256, atomic
  0600-permission write) gating the daemon's first unattended start via a
  new `setup` CLI subcommand.
- Default control-plane URL now https://amcp.home.jerryyan.top (env var
  override unchanged).
- Enrollment no longer requires a pre-issued token; falls back to
  zero-token self-service enrollment when none is configured.

Cloud control plane:
- CLOUD_SELF_SERVICE_ENROLLMENT_ENABLED (default false) opt-in flag.
- SelfServiceEnrollmentAuthProvider + ChainedEnrollmentAuthProvider:
  configured tokens still take priority; self-service only applies when
  no token matches, preserving edge-host-enrollment's token-bound path.
- Fixed a latent bug in sql_repository.py::enroll_host: the token-conflict
  lookup used `== enrollment_token_digest`, which SQLAlchemy compiles to
  `IS NULL` when the value is None, so every self-service enrollment after
  the first would have falsely collided with an existing NULL-digest host.
  Skipped that lookup entirely when the digest is None.

Docs/deploy: .env.example, compose.yaml, compose.deploy.yaml,
CLOUD_DEPLOYMENT.md, MACOS_IPHONE_SETUP.md updated for the new flag,
URL default, and required `device-host-agent setup` step.

Verification: 494 non-integration tests pass; openspec validate --strict
passes. PostgreSQL-backed contract tests and full manual end-to-end
verification were not run (no Postgres/Docker or reachable cloud-api in
this environment); noted as unchecked in tasks.md 7.2/7.4.

packages/cloud-platform/cloud/auth.py

@@ -99,7 +99,12 @@ class EnrollmentCredential:
 @dataclass(frozen=True)
 class EnrollmentPrincipal:
     id: str
-    token_digest: str = field(repr=False)
+    token_digest: str | None = field(default=None, repr=False)
+
+
+@runtime_checkable
+class EnrollmentAuthProvider(Protocol):
+    def authenticate(self, request: object) -> EnrollmentPrincipal | None: ...
 
 
 class ConfiguredBearerAuthProvider:
@@ -156,6 +161,25 @@ class ConfiguredEnrollmentTokenProvider:
         return matched
 
 
+class SelfServiceEnrollmentAuthProvider:
+    """Unconditionally authorizes Host enrollment with no pre-issued token."""
+
+    def authenticate(self, request: object) -> EnrollmentPrincipal | None:
+        return EnrollmentPrincipal(id="self-service", token_digest=None)
+
+
+class ChainedEnrollmentAuthProvider:
+    def __init__(self, providers: Iterable[EnrollmentAuthProvider]) -> None:
+        self.providers = tuple(providers)
+
+    def authenticate(self, request: object) -> EnrollmentPrincipal | None:
+        for provider in self.providers:
+            principal = provider.authenticate(request)
+            if principal is not None:
+                return principal
+        return None
+
+
 class RepositoryHostAuthProvider:
     def __init__(self, repository: CloudRepository) -> None:
         self.repository = repository