Hot-reload sshHosts/knownDevices from hosts.yaml, no restart needed

Checks the file's mtime on each poll cycle (already running every
30s) rather than adding a separate file-watcher or admin UI. Proxmox
hosts already needed no config (auto-discovered every poll); this
covers the two lists that did. A parse failure logs and keeps the
previous config running instead of crashing the poller.

Closes #14.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/hot-reload.md

@@ -0,0 +1,18 @@
+# Editing monitored hosts without a redeploy
+
+`config/hosts.yaml`'s `sshHosts` and `knownDevices` lists are re-read on the next poll
+cycle (checks the file's mtime, reloads if changed) — no container restart needed.
+
+- **Proxmox-monitored hosts** (the node + every LXC) need no config at all — they're
+  auto-discovered from the Proxmox API on every poll already.
+- **SSH-monitored hosts** (`sshHosts`): edit `config/hosts.yaml` on the deploy host
+  directly (`/opt/homelab-monitor/config/hosts.yaml` on CT122), save, wait up to one
+  poll interval (`POLL_INTERVAL_SECONDS`, default 30s). Adding a *new* SSH host still
+  needs a one-time manual step first — installing the forced-command key on the target
+  (see `docs/ssh-collector-key-setup.md`) — that part can't be automated away, so a web
+  form wouldn't make this fully self-service regardless of hot-reload.
+- **`knownDevices`** (device discovery labeling): same file, same reload path, no
+  manual step needed since it's just a label list.
+
+If the file fails to parse (bad YAML), the error is logged and the **previous**
+in-memory config keeps running rather than crashing the poller.