fix: advertise new sessions at first turn start, not after it completes

[william0wang]

Problem

A brand-new conversation only became visible to remote clients (phone app, second editor) after its first turn fully completed — markSessionActive fired only in prompt()'s finally block. A minutes-long first turn meant the session was invisible in remote discovery lists for minutes.

Fix

• Mark the session active at turn start (right after the preempt-lock registration), so it appears in remote lists within one heartbeat of the first prompt — even while that turn is still running.
• Seed a provisional title (first line of the prompt, capped at 60 chars) so the list entry isn't titleless. The backend's end-turn auto-title stays authoritative — its set-once gate is the separate server.sessionTitles set, so provisional titles never conflict with it.

Verification

• Live-tested with an isolated-ports discovery probe: new session appeared in the discovery list at 8.0s while its first turn was still running (turn settled at 8.9s); the backend auto-title then won via heartbeat enrichment.
• pnpm typecheck, pnpm lint, pnpm test all green (626/626).

Docs

• docs/REMOTE-CLIENTS.md: discovery contract updated (first turn start + provisional titles)
• CHANGELOG.md: [Unreleased] Fixed entry