feat: move session sharing and collab rooms into a plugin (B12)

plugins/session-sharing/README.md

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+# Session Sharing
+
+Share a live terminal or remote desktop session by link or with another user,
+and present sessions to a group in collaboration rooms.
+
+Bundled with Termix and enabled by default. It imports only
+`@termix/plugin-sdk`.
+
+## What it does
+
+- **Shares**: a link for anonymous guests, or a grant for one user, read-only
+  or read-write, with an expiry. The share button sits in the terminal and
+  remote desktop toolbars.
+- **Rooms**: members watch one stage, the live session the presenter shows.
+  The presenter or host can hand input control to a member, and a room can
+  have an anonymous guest link.
+- **`sessions.sharing`**: member joins and room events for the terminal
+  socket. Guest joins go through `sessions.sharing.guests` on `ctx.registry`,
+  because a guest has no user for a service's permission check.
+
+It reaches live sessions only through the `sessions.live` service, keyed by
+session type: ssh-terminal provides `ssh`, remote desktop provides `rdp`,
+`vnc` and `telnet`.
+
+## Layout
+
+    manifest.json         id, capabilities and what this plugin contributes
+    src/backend/          activate(ctx), routes, the room hub, the services
+    src/frontend/         share dialog, guest views, rooms panel and tab
+    migrations/           one .sql per dialect, generated from src/backend/tables.ts
+    tests/                backend (node) and frontend (jsdom)