fix: [nomad-prep] P5 — add healthchecks to agents, edge, staging, woodpecker-agent (#794)

Add Docker healthcheck blocks so Nomad check stanzas map 1:1 at migration:

- agents / agents-llama: pgrep -f entrypoint.sh (60s interval)
- woodpecker-agent: wget healthz on :3333 (30s interval)
- edge: curl Caddy admin API on :2019 (30s interval)
- staging: wget Caddy admin API on :2019 (30s interval)
- chat: add /health endpoint to server.py (no-auth 200 OK), fix
  Dockerfile HEALTHCHECK to use it, add compose-level healthcheck

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/chat/server.py

@@ -481,6 +481,14 @@ class ChatHandler(BaseHTTPRequestHandler):
         parsed = urlparse(self.path)
         path = parsed.path
 
+        # Health endpoint (no auth required) — used by Docker healthcheck
+        if path == "/health":
+            self.send_response(200)
+            self.send_header("Content-Type", "text/plain")
+            self.end_headers()
+            self.wfile.write(b"ok\n")
+            return
+
         # Verify endpoint for Caddy forward_auth (#709)
         if path == "/chat/auth/verify":
             self.handle_auth_verify()