[Feat] Start empty Fast Sessions

[roomote-roomote]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Allowed the existing New Session composer to submit with no message, using the same Send button and Enter shortcut already used for prompts.
• Routed empty submissions through the canonical Fast conversation get-or-create path with a stable client identity, then skipped the kickoff turn.
• Persisted empty Sessions as manual starts so the Session view renders idle after reload and remains ready for its first message.
• Preserved the existing composer UI and ordinary prompt/attachment-first Session creation.

Why this change was made

Opening the New Session composer did not persist a Session until a first message was sent. The previous empty-task behavior supported establishing the workspace first without requiring a separate visible action.

Impact

Clicking Send or pressing Enter with an empty composer now creates and enters one canonical idle Fast Session without triggering inference. Duplicate submissions and response-loss retries converge on the same Session, while non-empty submissions continue to schedule the normal first turn.

Validation passed across 66 focused client tests, 34 focused server tests, web TypeScript, web ESLint, formatting, and the full pre-push oxlint, residual lint, fast typecheck, and knip gates.

Screenshots

[roomote-community]

No new code issues found. See task

_{Reviewed c63184f}

[mrubens]

@roomote-roomote I don't want the "start empty session" button, just for it to be kind of a hidden feature if you press the send button without a message (the way it used to work for tasks)

[roomote-roomote]

@roomote-roomote I don't want the "start empty session" button, just for it to be kind of a hidden feature if you press the send button without a message (the way it used to work for tasks)

I’ll replace the visible action with the old task-style behavior: an empty composer can be submitted via the existing send control to create the Session, with no additional UI.

I’ve updated the implementation direction on PR #2235: no visible button; submitting the empty composer through the existing send control creates an idle Fast Session without inference. I’ll update the tests and PR description accordingly.

Reply with @-mention or use the web app.

[roomote-roomote]

Updated the existing composer flow in c63184f: there is no additional visible action, and submitting the empty composer with Send or Enter creates an idle Fast Session without inference. Focused tests and all relevant static/pre-push checks passed. See task.