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_auth.py

@@ -7,6 +7,7 @@ import pytest
 from cloud.auth import (
     BearerCredential,
     ChainedAuthProvider,
+    ChainedEnrollmentAuthProvider,
     ConfiguredBearerAuthProvider,
     ConfiguredEnrollmentTokenProvider,
     EnrollmentCredential,
@@ -14,6 +15,7 @@ from cloud.auth import (
     HostPrincipalRequiredError,
     NullAuthProvider,
     RepositoryHostAuthProvider,
+    SelfServiceEnrollmentAuthProvider,
     digest_token,
 )
 
@@ -213,3 +215,67 @@ def test_repository_host_auth_and_chain_preserve_host_scope() -> None:
     assert host_principal is not None
     assert host_principal.host_id == "host-managed"
     assert host_principal.scopes == frozenset()
+
+
+def test_self_service_enabled_with_no_token_enrolls_via_self_service() -> None:
+    provider = ChainedEnrollmentAuthProvider(
+        [ConfiguredEnrollmentTokenProvider(()), SelfServiceEnrollmentAuthProvider()]
+    )
+
+    principal = provider.authenticate(_Request(headers={}))
+
+    assert principal is not None
+    assert principal.id == "self-service"
+    assert principal.token_digest is None
+
+
+def test_self_service_disabled_with_no_token_is_unauthenticated() -> None:
+    provider = ChainedEnrollmentAuthProvider([ConfiguredEnrollmentTokenProvider(())])
+
+    assert provider.authenticate(_Request(headers={})) is None
+
+
+def test_self_service_enabled_with_valid_configured_token_uses_token_bound_path() -> (
+    None
+):
+    credential = EnrollmentCredential(
+        principal_id="installer-a",
+        token="one-time-enrollment-secret",
+    )
+    provider = ChainedEnrollmentAuthProvider(
+        [
+            ConfiguredEnrollmentTokenProvider([credential]),
+            SelfServiceEnrollmentAuthProvider(),
+        ]
+    )
+
+    principal = provider.authenticate(
+        _Request(headers={"authorization": "Bearer one-time-enrollment-secret"})
+    )
+
+    assert principal is not None
+    assert principal.id == "installer-a"
+    assert principal.token_digest == digest_token("one-time-enrollment-secret")
+
+
+def test_self_service_enabled_with_unknown_token_falls_through_to_self_service() -> (
+    None
+):
+    credential = EnrollmentCredential(
+        principal_id="installer-a",
+        token="one-time-enrollment-secret",
+    )
+    provider = ChainedEnrollmentAuthProvider(
+        [
+            ConfiguredEnrollmentTokenProvider([credential]),
+            SelfServiceEnrollmentAuthProvider(),
+        ]
+    )
+
+    principal = provider.authenticate(
+        _Request(headers={"authorization": "Bearer unknown-secret"})
+    )
+
+    assert principal is not None
+    assert principal.id == "self-service"
+    assert principal.token_digest is None