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/control_config.py

@@ -32,6 +32,7 @@ class CloudControlConfig:
     allow_insecure_anonymous: bool = False
     credentials: tuple[BearerCredential, ...] = ()
     enrollment_credentials: tuple[EnrollmentCredential, ...] = ()
+    self_service_enrollment_enabled: bool = False
     cors_allowed_origins: tuple[str, ...] = ()
     console_static_dir: str | None = None
     user_session_idle_seconds: int = 28_800
@@ -99,6 +100,10 @@ def load_control_config(
         enrollment_credentials=_parse_enrollment_credentials(
             values.get("CLOUD_ENROLLMENT_TOKENS_JSON")
         ),
+        self_service_enrollment_enabled=_parse_bool(
+            values.get("CLOUD_SELF_SERVICE_ENROLLMENT_ENABLED"),
+            default=False,
+        ),
         cors_allowed_origins=_parse_cors_origins(
             values.get("CLOUD_CONSOLE_CORS_ORIGINS")
         ),