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/sql_repository.py

@@ -45,7 +45,7 @@ class SQLAlchemyCloudRepository:
         host_id: str,
         agent_instance_id: str,
         credential_digest: str,
-        enrollment_token_digest: str,
+        enrollment_token_digest: str | None,
         display_name: str | None,
         enrolled_at: datetime,
     ) -> Any:
@@ -72,11 +72,19 @@ class SQLAlchemyCloudRepository:
                         )
                     return _host_enrollment_from_row(existing)
 
-                token_owner = session.scalars(
-                    select(HostRow).where(
-                        HostRow.enrollment_token_digest == enrollment_token_digest
-                    )
-                ).first()
+                # A NULL enrollment_token_digest marks self-service Hosts, which
+                # never bind a shared token; comparing `== NULL` would otherwise
+                # match every other self-service Host via SQL's `IS NULL` and
+                # falsely report a token conflict.
+                token_owner = (
+                    session.scalars(
+                        select(HostRow).where(
+                            HostRow.enrollment_token_digest == enrollment_token_digest
+                        )
+                    ).first()
+                    if enrollment_token_digest is not None
+                    else None
+                )
                 if token_owner is not None:
                     raise EnrollmentTokenConflictError(
                         "enrollment token is already bound to another Host"
@@ -109,11 +117,15 @@ class SQLAlchemyCloudRepository:
                     and existing.enrollment_token_digest == enrollment_token_digest
                 ):
                     return _host_enrollment_from_row(existing)
-                token_owner = session.scalars(
-                    select(HostRow).where(
-                        HostRow.enrollment_token_digest == enrollment_token_digest
-                    )
-                ).first()
+                token_owner = (
+                    session.scalars(
+                        select(HostRow).where(
+                            HostRow.enrollment_token_digest == enrollment_token_digest
+                        )
+                    ).first()
+                    if enrollment_token_digest is not None
+                    else None
+                )
             if token_owner is not None:
                 raise EnrollmentTokenConflictError(
                     "enrollment token is already bound to another Host"