feat: one-shot session titles with remote rename endpoint

[william0wang]

Summary

• One-shot title: a session is titled exactly once, from its first prompt, the moment it is sent (previously: on first end_turn, which a preempting second message could steal — the interrupted first turn ends cancelled and never reached the title block). After the one-shot, no automatic path revises a title; manual rename is the only later modifier.
• Rename endpoint: POST /api/instances/{id}/sessions/{sessionId}/rename (JSON body {"title"}), byte-proxied by the hub next to the existing close route. The bridge pins the title (title_overridden=1 in the App's tasks-index — same marker the App's own rename sets), updates discovery, and broadcasts session_info_update so attached editors update live.
• Bug fix: updateSessionTitle no longer writes the auto title into meta_json.title of a user-renamed row (the App may read either field; the write could visually revert the user's rename).
• Side benefit: titles now reach the editor, the ZCode App, and remote discovery immediately at the first prompt instead of after the first turn completes.

Test plan

• 6 new/rewritten title tests (tests/session-title.test.ts): one-shot at first prompt, preempt cannot steal, later prompts change nothing, resumed sessions never auto-titled, multi-line/80-char truncation
• 4 rename endpoint tests + 1 hub proxy body-relay test + tasks-index overridden-branch assertion updated
• Full suite 706/706, typecheck, lint, prettier