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.

apps/device-host-agent/host_agent/enrollment.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 from dataclasses import replace
 
 from host_agent.client import HostAgentEnrollmentClient
-from host_agent.config import HostAgentConfig, HostAgentConfigurationError
+from host_agent.config import HostAgentConfig
 from host_agent.identity import HostIdentityStore
 
 
@@ -17,10 +17,6 @@ def resolve_host_identity(
         return config
     state = identity_store.load_or_create()
     if state.host_id is None:
-        if not config.enrollment_token:
-            raise HostAgentConfigurationError(
-                "HOST_AGENT_ENROLLMENT_TOKEN is required to complete enrollment"
-            )
         response = client.enroll_host(
             agent_instance_id=state.agent_instance_id,
             host_token=state.token,