feat(gateway): add tailscale auth + pam

docs/tailscale.md

@@ -0,0 +1,87 @@
+---
+summary: "Integrated Tailscale Serve/Funnel for the Gateway dashboard"
+read_when:
+  - Exposing the Gateway Control UI outside localhost
+  - Automating tailnet or public dashboard access
+---
+# Tailscale (Gateway dashboard)
+
+Clawdis can auto-configure Tailscale **Serve** (tailnet) or **Funnel** (public) for the
+Gateway dashboard and WebSocket port. This keeps the Gateway bound to loopback while
+Tailscale provides HTTPS, routing, and (for Serve) identity headers.
+
+## Modes
+
+- `serve`: Tailnet-only HTTPS via `tailscale serve`. The gateway stays on `127.0.0.1`.
+- `funnel`: Public HTTPS via `tailscale funnel`. Requires auth.
+- `off`: Default (no Tailscale automation).
+
+## Auth
+
+Set `gateway.auth.mode` to control the handshake:
+
+- `token` (default when `CLAWDIS_GATEWAY_TOKEN` is set)
+- `password` (shared secret via `CLAWDIS_GATEWAY_PASSWORD` or config)
+- `system` (PAM, validates your OS password)
+
+When `tailscale.mode = "serve"`, the gateway trusts Tailscale identity headers by
+default unless you force `gateway.auth.mode` to `password`/`system` or set
+`gateway.auth.allowTailscale: false`.
+
+## Config examples
+
+### Tailnet-only (Serve)
+
+```json5
+{
+  gateway: {
+    bind: "loopback",
+    tailscale: { mode: "serve" }
+  }
+}
+```
+
+Open: `https://<magicdns>/ui/`
+
+### Public internet (Funnel + system password)
+
+```json5
+{
+  gateway: {
+    bind: "loopback",
+    tailscale: { mode: "funnel" },
+    auth: { mode: "system" }
+  }
+}
+```
+
+Open: `https://<magicdns>/ui/` (public)
+
+### Public internet (Funnel + shared password)
+
+```json5
+{
+  gateway: {
+    bind: "loopback",
+    tailscale: { mode: "funnel" },
+    auth: { mode: "password", password: "replace-me" }
+  }
+}
+```
+
+Prefer `CLAWDIS_GATEWAY_PASSWORD` over committing a password to disk.
+
+## CLI examples
+
+```bash
+clawdis gateway --tailscale serve
+clawdis gateway --tailscale funnel --auth system
+```
+
+## Notes
+
+- Tailscale Serve/Funnel requires the `tailscale` CLI to be installed and logged in.
+- System auth uses the optional `authenticate-pam` native module; install if missing.
+- `tailscale.mode: "funnel"` refuses to start without auth to avoid public exposure.
+- Set `gateway.tailscale.resetOnExit` if you want Clawdis to undo `tailscale serve`
+  or `tailscale funnel` configuration on shutdown.