feat: convert ssh tunnels into a plugin

plugins/tunnels/README.md

@@ -0,0 +1,44 @@
+# Tunnels
+
+SSH port forwarding between servers, and client tunnels from the desktop app.
+
+Bundled with Termix and enabled by default. It imports only
+`@termix/plugin-sdk`.
+
+## What it does
+
+- **Server tunnels**: local, remote and dynamic (SOCKS5) forwards saved on a
+  host, started from the Tunnels tab, the host editor or at boot when marked
+  auto-start. A tunnel either rides the source host's connection alone, or
+  opens a second SSH leg to a Termix endpoint host through a channel on the
+  source connection.
+- **Client tunnels**: the desktop app listens locally and relays each
+  connection over `/plugin-ws/tunnels/c2s/stream`; presets of that list are
+  saved per user.
+- **`tunnels.access`**: a service other plugins call. `forward()` opens an
+  on-demand forward and resolves once it works (web-endpoint uses it);
+  `start`/`stop`/`status`/`list` work on saved tunnels by name (automations).
+
+Both SSH legs go through `ctx.ssh.connect`, so tunnels get host key checks,
+auth providers, proxies and jump hosts like every other transport. The
+endpoint leg passes its forwardOut channel as `sock`.
+
+## Layout
+
+    manifest.json        id, capabilities and what this plugin contributes
+    src/backend/         activate(ctx), the tunnel manager, routes, relay, service
+    src/shared/          types and tunnel naming used by both bundles
+    migrations/          one .sql per dialect, generated from src/backend/tables.ts
+    src/frontend/        the tab, host editor section, rail panel and widget
+    locales/en.json      its strings (English only; the rest are translated)
+    tests/backend/       vitest, node
+    tests/frontend/      vitest, jsdom
+
+## Data
+
+- `p_tunnels_presets`, adopted from core's `c2s_tunnel_presets`.
+- Host settings `enableTunnel` and `tunnelConnections`, copied from the
+  `ssh_data` columns of the same names by core's
+  `tunnels-settings-migration.ts`.
+
+See packages/plugin-sdk/ARCHITECTURE.md for the contract this follows.