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.

tests/test_cloud_repository_contract.py

@@ -159,6 +159,74 @@ def test_host_enrollment_is_idempotent_and_token_is_one_time(
         database.close()
 
 
+def test_self_service_enrollment_is_idempotent_with_null_token_digest(
+    database_url: str,
+) -> None:
+    database = CloudDatabase(database_url)
+    repository = database.repository
+    enrolled_at = datetime(2026, 7, 13, 6, 0, tzinfo=UTC)
+    host_id = _unique_id("self-service-host")
+    agent_instance_id = _unique_id("self-service-instance")
+    credential_digest = _unique_id("self-service-credential")
+
+    try:
+        created = repository.enroll_host(
+            host_id=host_id,
+            agent_instance_id=agent_instance_id,
+            credential_digest=credential_digest,
+            enrollment_token_digest=None,
+            display_name="Self-Service Host",
+            enrolled_at=enrolled_at,
+        )
+        assert created.host_id == host_id
+
+        retried = repository.enroll_host(
+            host_id=_unique_id("ignored-host"),
+            agent_instance_id=agent_instance_id,
+            credential_digest=credential_digest,
+            enrollment_token_digest=None,
+            display_name="Renamed Self-Service Host",
+            enrolled_at=enrolled_at + timedelta(minutes=1),
+        )
+        assert retried == created
+    finally:
+        database.close()
+
+
+def test_multiple_self_service_hosts_coexist_without_token_conflict(
+    database_url: str,
+) -> None:
+    database = CloudDatabase(database_url)
+    repository = database.repository
+    enrolled_at = datetime(2026, 7, 13, 7, 0, tzinfo=UTC)
+    credential_a = _unique_id("self-service-credential-a")
+    credential_b = _unique_id("self-service-credential-b")
+
+    try:
+        first = repository.enroll_host(
+            host_id=_unique_id("self-service-host-a"),
+            agent_instance_id=_unique_id("self-service-instance-a"),
+            credential_digest=credential_a,
+            enrollment_token_digest=None,
+            display_name=None,
+            enrolled_at=enrolled_at,
+        )
+        second = repository.enroll_host(
+            host_id=_unique_id("self-service-host-b"),
+            agent_instance_id=_unique_id("self-service-instance-b"),
+            credential_digest=credential_b,
+            enrollment_token_digest=None,
+            display_name=None,
+            enrolled_at=enrolled_at,
+        )
+
+        assert first.host_id != second.host_id
+        assert repository.authenticate_enrolled_host(credential_a) == first.host_id
+        assert repository.authenticate_enrolled_host(credential_b) == second.host_id
+    finally:
+        database.close()
+
+
 def test_device_enrollment_is_host_scoped_and_idempotent(
     database_url: str,
 ) -> None: