feat(groups): session Claude unique par groupe via --add-dir (P6)

Un groupe lance désormais UNE seule session Claude couvrant tous ses
repos/worktrees (--add-dir) au lieu d'une session par repo, pour
travailler en simultané dans une conversation partagée. Le transport WS,
le flow control et l'attach sont inchangés (toujours 1 session, 1 channel).

- shared: SessionSummary.addedDirs/groupId (champs additifs optionnels) ;
  CreateGroupSessionRequest / GroupSessionResponse.
- server: --add-dir dans claude-launcher ; spawn({addDirs,groupId}) +
  persistance (migration #8 : sessions.added_dirs/group_id) ;
  POST /api/v1/groups/:id/session (résolution des dirs cote serveur) ;
  resume re-relie les dirs ; deleteGroup nullifie group_id.
- web: GroupSessionModal (modes feature / checkouts principaux) ;
  createGroupFeature/createGroupSession ; badge « groupe · N depots » ;
  sessionsInGroup dedupliquee.
- site: textes EN/FR « une seule session Claude » + diagramme hub.
- tests: claude-launcher, pty-manager (groupe), group-manager ;
  acceptation P5 etendue (session de groupe multi-repo). 272 tests verts.

packages/site/src/i18n/en.ts

@@ -29,7 +29,7 @@ export default {
   feat6Title: 'Answer without a terminal',
   feat6Desc: 'Respond to agent prompts and dialogs right from the dashboard.',
   feat7Title: 'Work groups',
-  feat7Desc: 'Group repos and launch the same feature across all of them at once.',
+  feat7Desc: 'Group repos and run one Claude session across all of them at once.',
   feat8Title: 'Local-first & secure',
   feat8Desc: 'Binds to localhost, hashes tokens, remote access via Tailscale.',
   scAKicker: 'Worktree-first dashboard',
@@ -44,10 +44,11 @@ export default {
   scCTitle: 'Unblock an agent from the kitchen',
   scCBody:
     'Install the PWA and get a Web Push the second a session needs you. Tap the notification, read the prompt, answer the dialog — your agent keeps moving while you are away.',
+  grpHub: '1 session',
   grpKicker: 'Work groups · cross-repo',
-  grpTitle: 'Ship one feature across every repo at once',
+  grpTitle: 'Drive every repo from one Claude session',
   grpBody:
-    'Group the repos that move together — a service, its client, its docs. Launch the same task to all of them and Arboretum spins up a worktree and an agent in each, then tracks them as one unit.',
+    'Group the repos that move together — a service, its client, its docs. Launch a single Claude session that spans all of them at once: one conversation, one shared context working across every repo — instead of juggling one agent per repo.',
   howKicker: 'How it works',
   howTitle: 'Zero install. Three steps.',
   step1Title: 'Launch the daemon',