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.

compose.deploy.yaml

@@ -28,6 +28,11 @@ services:
       CLOUD_PUBLIC_CREDENTIALS_JSON: ${CLOUD_PUBLIC_CREDENTIALS_JSON}
       CLOUD_HOST_CREDENTIALS_JSON: ${CLOUD_HOST_CREDENTIALS_JSON}
       CLOUD_ENROLLMENT_TOKENS_JSON: ${CLOUD_ENROLLMENT_TOKENS_JSON:-[]}
+      # Left disabled by default; the amcp.home.jerryyan.top deployment sets
+      # CLOUD_SELF_SERVICE_ENROLLMENT_ENABLED=true in its own .env (not
+      # committed here) since it is the intended target for zero-token edge
+      # enrollment. See docs/CLOUD_DEPLOYMENT.md.
+      CLOUD_SELF_SERVICE_ENROLLMENT_ENABLED: ${CLOUD_SELF_SERVICE_ENROLLMENT_ENABLED:-false}
       CLOUD_SCHEDULER_INTERVAL_SECONDS: ${CLOUD_SCHEDULER_INTERVAL_SECONDS:-1}
       CLOUD_LEASE_REAPER_INTERVAL_SECONDS: ${CLOUD_LEASE_REAPER_INTERVAL_SECONDS:-5}
       CLOUD_LEASE_DURATION_SECONDS: ${CLOUD_LEASE_DURATION_SECONDS:-60}