feat(vscode): extension VS Code native (intégration native, pas un webview)

Nouveau workspace packages/vscode (git-arboretum, privé, non publié sur npm), client REST/WS
réutilisant @arboretum/shared. Auth Authorization: Bearer sur REST et l'upgrade WS (via `ws`) ;
un client Node sans en-tête Origin passe le check Origin strict du serveur.

- Arbres temps réel : Repositories (repos → worktrees → sessions) et Groups, via le WebSocket.
- Terminaux natifs (vscode.Pseudoterminal) pour attacher/observer une session — rendu et
  scrollback de VS Code ; décodage UTF-8 streaming + comptabilité ACK dans des modules purs.
- Status bar (compteur waiting) + notifications natives sur passage en waiting, réponses Yes/No
  via la commande WS answer.
- Mutations git (create worktree, commit, push, promote), start/kill/hide/resume/fork, session
  de groupe ; conscience du workspace (reveal + start/create here).
- Bundle esbuild (format cjs, external vscode) inlinant @arboretum/shared → VSIX autonome.
  Logique réutilisable sans import vscode → testée par vitest (19 tests).
- CI : .gitea/workflows/vscode-release.yml package le VSIX sur tag vscode-vX.Y.Z (artefact +
  asset de release best-effort). build:vscode hors du build principal (comme le site).
- spikes/s5-vscode/STUDY.md : décision de conception (GO phasé A→D), marquée implémentée.

packages/vscode/README.md

@@ -0,0 +1,73 @@
+# Arboretum for VS Code
+
+Pilot your git **worktrees** and **Claude Code sessions** from inside VS Code — a real native
+integration on top of the [Arboretum](https://git.lidge.fr/johanleroy/arboretum) daemon, not a webview.
+
+- **Live tree** of Repositories → Worktrees → Sessions (and a Groups view), updated in real time over
+  the daemon's WebSocket.
+- **Native terminals**: attach to any session in a real VS Code terminal (a `Pseudoterminal` bridges the
+  daemon's PTY) — you get VS Code's own rendering, scrollback, copy/paste and links for free.
+- **Waiting alerts**: a status-bar counter and native notifications when a Claude session is waiting for
+  input — answer **Yes/No** without even opening the terminal (uses the daemon's `answer` command).
+- **Git mutations** from the tree: create worktree, commit, push, promote to main.
+- **Workspace-aware**: the worktree matching your open folder is highlighted; start a session or create a
+  worktree for the current folder in one command.
+
+## Requirements
+
+- A running Arboretum daemon (`npx @johanleroy/git-arboretum`, or installed as a user service via
+  `arboretum install`). The extension is a **client** — it does not start the daemon.
+- An access token. The bootstrap token is printed once on first daemon start; you can also create one in
+  the Arboretum dashboard (**Settings → Tokens**).
+
+## Getting started
+
+1. Start the daemon and copy a token.
+2. In VS Code, run **Arboretum: Sign In** (Command Palette) and paste the token. The token is validated
+   against the daemon and stored in VS Code's encrypted **SecretStorage**.
+3. The **Arboretum** view container appears in the Activity Bar with the Repositories and Groups trees.
+
+## Settings
+
+| Setting | Default | Description |
+| --- | --- | --- |
+| `arboretum.url` | `http://127.0.0.1:7317` | Base URL of the daemon (REST + WebSocket). |
+| `arboretum.showExternalSessions` | `false` | Also show Claude sessions started outside Arboretum (CLI). |
+| `arboretum.notifyOnWaiting` | `true` | Native notification when a session starts waiting for input. |
+
+The extension authenticates with `Authorization: Bearer <token>` on both REST and the WebSocket upgrade.
+For remote access, point `arboretum.url` at your Tailscale Serve URL (`https://…`) — the WebSocket is
+derived automatically (`wss://`).
+
+## Building & packaging (private VSIX)
+
+The extension is bundled with esbuild (`@arboretum/shared` is inlined → the VSIX is self-contained).
+
+```bash
+# from the monorepo root
+npm install
+npm run build:vscode                       # builds @arboretum/shared then bundles the extension
+cd packages/vscode && npx @vscode/vsce package --no-dependencies
+# → git-arboretum-0.1.0.vsix
+```
+
+Install it with **Extensions: Install from VSIX…** in the Command Palette, or:
+
+```bash
+code --install-extension git-arboretum-0.1.0.vsix   # also: codium / cursor
+```
+
+### Other distribution channels (optional)
+
+- **Open VSX** (VSCodium, Cursor, Windsurf): `npx ovsx publish *.vsix -p <token>` after creating an
+  Open VSX account and namespace.
+- **VS Code Marketplace**: create an Azure DevOps publisher + PAT, then `npx @vscode/vsce publish`.
+
+A Gitea Actions workflow packages the VSIX automatically on a `vscode-vX.Y.Z` tag (see
+`.gitea/workflows/vscode-release.yml`) and attaches it to the release.
+
+## Security
+
+The extension never weakens the daemon's security model: it speaks the same authenticated protocol as the
+web dashboard. A Node client sends no `Origin` header, so it passes the daemon's strict Origin check while
+still requiring a valid token.