feat(updater): remote self-update daemon with auto-rollback

Adds a separate `decnet updater` daemon on each worker that owns the
agent's release directory and installs tarball pushes from the master
over mTLS. A normal `/update` never touches the updater itself, so the
updater is always a known-good rescuer if a bad agent push breaks
/health — the rotation is reversed and the agent restarted against the
previous release. `POST /update-self` handles updater upgrades
explicitly (no auto-rollback).

- decnet/updater/: executor, FastAPI app, uvicorn launcher
- decnet/swarm/updater_client.py, tar_tree.py: master-side push
- cli: `decnet updater`, `decnet swarm update [--host|--all]
  [--include-self] [--dry-run]`, `--updater` on `swarm enroll`
- enrollment API issues a second cert (CN=updater@<host>) signed by the
  same CA; SwarmHost records updater_cert_fingerprint
- tests: executor, app, CLI, tar tree, enroll-with-updater (37 new)
- wiki: Remote-Updates page + sidebar + SWARM-Mode cross-link

pyproject.toml

@@ -24,7 +24,8 @@ dependencies = [
     "sqlmodel>=0.0.16",
     "scapy>=2.6.1",
     "orjson>=3.10",
-    "cryptography>=46.0.7"
+    "cryptography>=46.0.7",
+    "python-multipart>=0.0.20"
 ]
 
 [project.optional-dependencies]