fix(harness): persist session title across client reloads

[OmGuptaIND]

Summary

• Add `writeHarnessSessionTitle()` in `agent-core/src/harness/mirror.ts` to persist a harness session title to `meta.json`, and wire it into the server's `set_session_title` handler so the model's chosen title survives a client reload (the `title_update` event only reaches connected clients).
• In `onTurnEnd`, prefer `session.getTitle()` over the assistant's first text snippet when seeding the title, so `meta.json` no longer latches onto AI prose like "I'm checking your Google Calendar accounts…".
• Read back through `getTitle()` after `setTitle()` so disk and memory stay in lockstep with the session's own normalization rules.

Test plan

• Start a harness session, let the model call `set_session_title`, reload the client — title persists.
• Start a harness session, do not call `set_session_title`, run a turn — `meta.json` still gets a sensible title (first text fallback).
• Call `set_session_title` mid-session after turn 0 has already written a fallback title — new title overwrites the fallback in `meta.json`.

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