Render thread references inline in the composer

[jonathanKingston]

Summary

• insert selected @thread references as atomic inline composer chips at the caret
• reuse Copse's existing thread and close SVGs plus the sent-thread outline geometry
• preserve chip position through draft switches and render it inline after send
• keep thread labels in agent-facing prose while retaining the structured thread reference
• add focused unit, browser-geometry, Electron E2E, screenshot, and prototype coverage

Validation

• pnpm run typecheck — passed after rebasing onto current origin/main
• pnpm run lint — passed
• pnpm run format:check — passed
• pnpm test -- composer-editor conversation-transcript input-bar modern-css — 101 passed
• pnpm run build:demo — passed
• pnpm exec wdio run wdio.demo.conf.ts --spec tests/demo/thread-reference-inline.demo.ts — passed in real Chrome; verifies matching outline tokens, baseline alignment, icon centering, and stationary hover
• static check phases for demo consistency, dead code, oracle invariants, E2E syntax, and E2E exclusions — passed
• focused Electron E2E is authored and syntax-clean, but was not run locally: the local launcher was rejected by the safety gate and no configured remote E2E destination was available
• the monolithic full unit tier exceeded the tool's five-minute foreground ceiling; its sandbox attempt exposed only five loopback preview-server EPERM failures, so hosted CI remains the full-tier authority

Visual evidence

• tests/e2e/screenshots/thread-reference-inline-composer.png

Co-Authored-By: Copse noreply@copse.dev
Copse-Models: acp:codex-acp#gpt-5.6-sol, acp:claude-acp#claude-fable-5-1[1m]

[github-actions]

Screenshot evidence

Changed reference candidates for 38efdc22e5b8 are in the screenshot artifact from CI run 35773612051.
The artifact is retained for 14 days; its presence does not mean the visual changes have been accepted.

Review this evidence with the parent PR. No baseline-update PR was opened.
If the references should change, add update-screenshots to run a full refresh and open a PNG review PR, or commit the reviewed PNGs manually. Remove the label once the review PR is created.
Do not refresh references merely to absorb unrelated rendering drift.

[github-actions]

🖥️ PR preview

• Marketing site
• Browser demo
  Interactive, with a mocked backend — jump to a scenario:
• Builds a cupcake site
• Agent-proposed edits open the Changes panel
• Readable assistant responses
• Markdown list indentation
• Unattended container run
• Responsive composer footer
• Subagent display visual reference
• Subagent stays ungrouped beside reading tools
• Semantic search subagent markdown
• Light-theme approval with a bright accent
• Light-theme syntax, fills, and selection
• Grouped outside-sandbox command approval
• Remote desktop discovered-port list with one selected
• Inline thread reference chip geometry
• Settings scroll + sticky footer geometry
• Per-model reasoning / temperature / top-p
• Thread switch hydration notice
• Thread switch hydration failure
• Apple Development test profile
• Chat layout styling
  _{Static demo of the renderer against a mock API — the browser pane, terminal, and all main-process IPC are stubbed, so it never proves main-process behaviour. Rebuilt whenever a push changes the demo bundle; these links go live once the queued deploy lands, shortly after this comment. Removed when the PR closes.}