feat: groupes de travail (P5)

Ajoute la notion de groupe — collection nommée de repos (membership légère
persistée, many-to-many) — pour piloter plusieurs repos en simultané :

- DB : migration #5 (tables groups + group_repos, FK ON DELETE CASCADE).
- GroupManager (synchrone, EventEmitter) + routes REST /api/v1/groups ;
  ne persiste que la membership, worktrees/sessions filtrés par repoId côté client.
- Protocole : GroupSummary, DTOs, topic WS « groups », events group_update/removed
  (additifs, PROTOCOL_VERSION inchangé).
- Web : store groups, GroupsListView, GroupView (réutilise RepoSection),
  grille multi-terminaux (TerminalGrid/TerminalCell), action « feature cross-repo »
  (orchestration client, tolérante aux échecs partiels), routes + i18n EN/FR.
- Tests : group-manager.test.ts + extension protocol.test.ts ; acceptance-p5.mjs.

README.md

@@ -10,7 +10,7 @@
   <strong>English</strong> · <a href="README.fr.md">Français</a>
 </p>
 
-**Status: MVP.** The worktree-first dashboard, session discovery & resume, multi-repo worktree lifecycle, live session states, the web terminal, and mobile supervision (installable PWA, Web Push when a session needs you, approve/deny without opening a terminal) are implemented and tested.
+**Status: MVP.** The worktree-first dashboard, session discovery & resume, multi-repo worktree lifecycle, live session states, the web terminal, and mobile supervision (installable PWA, Web Push when a session needs you, approve/deny without opening a terminal), and work groups (run several related repos at once) are implemented and tested.
 
 ---
 
@@ -31,6 +31,7 @@ A single Node.js daemon you run on your dev machine (`npx @johanleroy/git-arbore
 - **Session discovery & resume** — sessions you launched in your own terminal show up automatically; resume dead ones, observe or fork live ones. Never corrupts a live session.
 - **Web terminal** — full xterm.js terminal to every managed session, surviving browser disconnects.
 - **Supervision from your phone** — installable PWA with push notifications when a session needs you; approve/deny a prompt without opening a terminal.
+- **Work groups** — bundle related repos (e.g. an API, its web frontend and a shared library) into a named group to work on them at once: a unified view of all their worktrees and sessions, a side-by-side multi-terminal grid, and a one-click "cross-repo feature" that creates the same branch worktree (and optionally a Claude session) across every repo in the group.
 
 ---
 
@@ -247,6 +248,7 @@ node packages/server/scripts/acceptance-p1.mjs   # core: daemon + real WS client
 node packages/server/scripts/acceptance-p2.mjs   # session discovery & resume
 node packages/server/scripts/acceptance-p3.mjs   # worktrees & session correlation
 node packages/server/scripts/acceptance-p4.mjs   # Web Push + WS `answer` command
+node packages/server/scripts/acceptance-p5.mjs   # work groups: CRUD + WS broadcast + CASCADE
 ```
 
 ## License