feat(chat): Part C — chat panel over the live /acp session

[brettchien]

Stacked on #48 (feat/chat-part-b-backend) — this PR's base is the Part B branch, so the diff is Part C only. Rebase onto main once #48 squash-merges (see the stacked-slice note).

What

The first visible, end-to-end chat: type a prompt → the agent streams back → rendered as markdown. Consumes the Part B backend (agent_prompt / agent_cancel commands + agent-update events).

Changes (console/)

• src/chat.ts — transcript view-model + pure render (mirrors render.ts). markdown-it (html:false, linkify) renders a finalized agent turn; spinner + raw text while streaming; user text and panel chrome are escaped. Pure reducers (appendUser / appendChunk / endTurn) keep the render a function of ChatTurn[], unit-testable without a DOM.
• src/main.ts — turn wiring. Single-shot turn model (katashiro): a prompt typed mid-turn is queued; flushQueue releases the next only when the current turn ends, so two turns never overlap (the backend in-flight guard from feat(chat): Part B(1) backend — session/prompt pipe over the live /acp socket #48 stays a safety net). agent-update (chunk/turn_end) drives the transcript; Stop → agent_cancel; per-turn Copy (raw text); a mid-turn socket drop finalizes the open turn so no spinner hangs. Agent markdown is DOMPurify-sanitized at the DOM-write boundary (markdown-it + DOMPurify, per ADR).
• src/source.ts — agentPrompt/agentCancel bridge; the browser build synthesizes a canned reply so the panel is demonstrable without a gateway.
• index.html + styles.css — the panel (transcript, input, Send/Stop), light/dark.
• src/chat.test.ts — 14 tests: markdown, raw-HTML / javascript:-link safety, reducers, render.

Verification

• tsc --noEmit clean, 59 console tests green, vite build OK.
• End-to-end against a live gateway needs the desktop shell (same limitation as Part B) — not runnable in CI.

Notes

• Bundle grows ~60 kB (markdown-it + dompurify); the >500 kB chunk warning is pre-existing (codemirror).