feat(cli): add --title flag to set tab title when summoning

Add a --title <title> flag to the dispatch CLI so a summoned agent's
conversation/tab gets a human-readable title at creation time instead of the
auto-derived default (Untitled until the first message append).

The title is carried as an additive optional ChatRequest.title field (the
HTTP client-server contract), parsed + trimmed server-side in parseChatBody,
and persisted via the conversation store's setConversationTitle in the POST
/chat route BEFORE the turn starts — so the title is set atomically with
creation and before --open signals the frontend to open the tab. No second
HTTP round-trip is needed.

A whitespace-only title is treated as absent (auto-derive); a non-string
title yields HTTP 400. A title-set failure is logged but never blocks the
turn (the title is a nicety, the answer is not). The change is purely
additive — no existing behavior or assertion changes.

Verification: typecheck EXIT 0; test 2021 passed | 6 skipped | 0 failed;
check EXIT 0 (12 pre-existing warnings in untouched files).

0 files changed, 0 insertions, 0 deletions