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/server/src/routes/groups.ts

@@ -2,16 +2,26 @@ import type { FastifyInstance } from 'fastify';
 import type {
   AddRepoRequest,
   CreateGroupRequest,
+  CreateGroupSessionRequest,
   GroupResponse,
+  GroupSessionResponse,
   GroupsListResponse,
   UpdateGroupRequest,
 } from '@arboretum/shared';
 import type { GroupManager } from '../core/group-manager.js';
+import type { WorktreeManager } from '../core/worktree-manager.js';
+import type { PtyManager } from '../core/pty-manager.js';
 import type { Db } from '../db/index.js';
 import { recordAudit } from '../core/audit-log.js';
 import { sendManagerError } from './repos.js';
 
-export function registerGroupRoutes(app: FastifyInstance, gm: GroupManager, db: Db): void {
+export function registerGroupRoutes(
+  app: FastifyInstance,
+  gm: GroupManager,
+  db: Db,
+  wt: WorktreeManager,
+  manager: PtyManager,
+): void {
   app.get('/api/v1/groups', async (): Promise<GroupsListResponse> => ({ groups: gm.listGroups() }));
 
   app.get('/api/v1/groups/:id', async (req, reply) => {
@@ -88,4 +98,62 @@ export function registerGroupRoutes(app: FastifyInstance, gm: GroupManager, db:
       return sendManagerError(reply, err);
     }
   });
+
+  // Session de groupe (P6) : UNE session Claude couvrant tous les repos du groupe (--add-dir).
+  // Les répertoires sont résolus côté serveur depuis les propres worktrees du groupe — le client
+  // ne passe jamais de chemin brut. `branch` présent → worktree de cette branche par repo ;
+  // absent → le worktree principal de chaque repo.
+  app.post('/api/v1/groups/:id/session', async (req, reply) => {
+    const { id } = req.params as { id: string };
+    const body = (req.body as Partial<CreateGroupSessionRequest> | null) ?? {};
+    const command = body.command ?? 'claude';
+    if (command !== 'claude' && command !== 'bash') {
+      return reply.status(400).send({ error: { code: 'BAD_REQUEST', message: 'command must be claude or bash' } });
+    }
+    const branch = typeof body.branch === 'string' && body.branch.trim() !== '' ? body.branch.trim() : null;
+
+    let group;
+    try {
+      group = gm.getGroup(id);
+    } catch (err) {
+      return sendManagerError(reply, err);
+    }
+
+    const dirs: string[] = [];
+    const skipped: Array<{ repoId: string; reason: string }> = [];
+    for (const repoId of group.repoIds) {
+      let worktrees;
+      try {
+        worktrees = await wt.listRepoWorktrees(repoId);
+      } catch (err) {
+        skipped.push({ repoId, reason: err instanceof Error ? err.message : String(err) });
+        continue;
+      }
+      const match = branch ? worktrees.find((w) => w.branch === branch) : worktrees.find((w) => w.isMain);
+      if (!match) {
+        skipped.push({ repoId, reason: branch ? `no worktree on branch ${branch}` : 'no main worktree' });
+        continue;
+      }
+      if (!dirs.includes(match.path)) dirs.push(match.path);
+    }
+
+    const [primary, ...rest] = dirs;
+    if (primary === undefined) {
+      return reply.status(400).send({ error: { code: 'BAD_REQUEST', message: 'no directory to span (group has no resolvable worktree)' } });
+    }
+
+    try {
+      const session = manager.spawn({ cwd: primary, addDirs: rest, command, groupId: id });
+      recordAudit(db, {
+        actor: req.authContext?.tokenId ?? 'unknown',
+        action: 'group.session.create',
+        resourceId: id,
+        details: { command, dirs: dirs.length, ...(branch ? { branch } : {}) },
+      });
+      return reply.status(201).send({ session, dirs, skipped } satisfies GroupSessionResponse);
+    } catch (err) {
+      const statusCode = (err as { statusCode?: number }).statusCode ?? 500;
+      return reply.status(statusCode).send({ error: { code: 'SPAWN_FAILED', message: (err as Error).message } });
+    }
+  });
 }

packages/server/src/routes/sessions.ts

@@ -39,7 +39,14 @@ export function registerSessionRoutes(app: FastifyInstance, manager: PtyManager,
       return reply.status(409).send({ error: { code: 'SESSION_LIVE', message: 'Session is live — fork it instead' } });
     }
     try {
-      const session = manager.spawn({ cwd: discovered.cwd, resume: { claudeSessionId: id } });
+      // Session de groupe (P6) : re-relie les mêmes répertoires (--add-dir) et son groupe au resume.
+      const ctx = manager.groupSessionContext(id);
+      const session = manager.spawn({
+        cwd: discovered.cwd,
+        resume: { claudeSessionId: id },
+        ...(ctx?.addedDirs.length ? { addDirs: ctx.addedDirs } : {}),
+        ...(ctx?.groupId ? { groupId: ctx.groupId } : {}),
+      });
       const res: SessionResponse = { session };
       return reply.status(201).send(res);
     } catch (err) {