bug(workspace): multi-agent chat picker is offered on builds with no rooms backend — 404 dead end

[trinity-ability]

Found while validating #2120 (ent#358/#361/#381), which merged the Workspace multi-agent chat.

What happens

The Workspace "+ New chat" picker (components/portal/PortalAgentPicker.vue) lets a client select several agents, captioned "they will share this conversation". Selecting 2+ routes to store.createRoom() → POST /api/rooms.

/api/rooms exists only in the enterprise submodule (shared_sessions). There is no OSS router with that prefix:

$ grep -rn 'prefix="/api/rooms"' src/backend --include='*.py'
src/backend/enterprise/backend/shared_sessions/router.py:29:    prefix="/api/rooms",

So on an OSS-only build — or any install without the shared_sessions entitlement — picking two agents produces a 404, and Portal.vue renders the generic 'Could not start that chat.' The affordance is always offered and silently cannot work.

Why it regressed

The surface this replaced was gated. NavBar.vue before #2120:

<router-link v-if="enterpriseStore.isEntitled('shared_sessions')" to="/sessions">

#2120 removed that entry (ent#381) and moved the capability into the picker, but no equivalent gate came with it. The new /workspace/r/:roomId route also carries no requiresEntitlement, unlike the /sessions/:roomId? route it replaced.

Why this is not a one-liner

isEntitled() reads enterprise_features from /api/settings/feature-flags, which requires platform auth. The Workspace is reachable by an external client on an email-OTP portal session with no platform JWT, so the flag is not currently available to the principal that needs it. Fixing this properly means exposing a rooms-availability signal to a portal principal (e.g. on the /my-agents roster payload or a portal-scoped flags read), then gating the multi-select on it.

Acceptance criteria

• On a build without shared_sessions, the picker does not offer multi-agent selection (or offers it visibly disabled with a reason) — never a dead-ended 404
• The gate works for an external client on a portal session, not only for a signed-in platform user
• Single-agent selection is unaffected in every edition
• /workspace/r/:roomId refuses/redirects when rooms are unavailable, rather than rendering an empty room

Related: #2120, abilityai/trinity-enterprise#361, abilityai/trinity-enterprise#381

[vybe]

/claim — picked up as part of a parallel orchestration run for the v0.9.0 freeze week (vybe).

[github-actions]

Claimed by @vybe. Label status-in-progress added.

Next steps:

1. Create branch: feature/2128-<short-slug>
2. Implement the feature
3. Open a PR with Fixes #2128