feat: update session thread root content once started

[kevin-david]

Replace the placeholder message with session details, enhancing user experience.

Tested locally in combination with #6

[0xmrpeter]

Nice UX improvement — the stale "⏳ Setting up..." placeholder has been a papercut for a while. The updateSessionThreadStarter implementation is clean and follows the forums.ts patterns well (defensive try/catch, log.warn on failure, consistent with renameSessionThread and deleteSessionThread).

One minor note: the format string 📂 **${agentName}** — `${workingDirectory}` is duplicated verbatim in both new-session.ts and adapter.ts. If the format ever changes, both sites need updating — a small shared helper exported from forums.ts (e.g. buildSessionStarterContent) would keep it in one place.

No blocking issues — good to go!