#26654 cmux NEXT composer: append Quote in Reply at the bottom

Leo: "I feel like putting it at the very lowest bit makes sense ... quote in reply ... a little bit useful"

The agent composer now accepts the shared hc-quote selection event and appends the selected text at the bottom of the existing draft. Each quote is separated by a blank line, successive quotes keep their order, and the editor focuses after the trailing blank line. Milkdown continues to render a quiet muted blockquote bar while serializing the prompt as Markdown (> ...).

• Added the shared composer quote append helper and event listener for composer, document, and window dispatch targets.
• Added bottom and stacked composer gallery fixtures while retaining Backspace unwrap coverage from #25681.
• Kept the native context-menu event boundary owned by the agent-chat surface; no Swift or transcript files changed.

Validation

• Red regression: a063ab5e12f (bun test test/composer-quote-backspace.test.tsx failed before the helper existed: module not found).
• Green repair: abf0afa9d17 (bun test test/composer-quote-backspace.test.tsx: 5 passed).
• bun run typecheck passed (2,896 files).
• Changed-file bunx vp fmt --check and bunx vp lint passed.
• python3 scripts/cmux-next/prepush-gate.py --base origin/feat-cmux-next --head HEAD passed.
• Browser gallery sweep on GCP passed: 40 tests, 157 expectations.
• No native build or local macOS verification; the exact quote visual is represented by the composer gallery bottom and stacked fixtures for the managed capture lane.

Changelog

• Changed: Quote in Reply appends muted Markdown quotes at the bottom of the agent composer draft.

Agent session: agent session

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Summary by cubic

Adds Quote in Reply support to the agent composer via the shared hc-quote event, appending quoted text to the bottom of the draft, separated by blank lines and preserving order.