#2940 feat(desktop): one agent pane for a task's agent and an agent's own page

Summary

Chain C, step C2a (items 10 and 11). The page of an agent an automation started (DesktopAgentPage) mounted AgentTab with nothing around it. It ran the full window width, ⌘F did nothing, an @ in its box offered nothing, and / had no skills. It now draws the task column's own agent pane, so a one-off agent reads, finds, and is written to the same way a task's agent is.

• AgentPane (main-tabs.tsx): the .desktop-agent reading-width section around MainTabContent. The task column now draws its agent and draft tabs through it, and the agent page draws its agent through it.
• ⌘F: the agent page wraps the pane in a FindScope. FindSubject gains a { agent } form beside { plan, tab }. C3 widens it to every page.
• @ with no plan: planlessMentions (plan-mentions.tsx) is the page's mention source. It offers the repository's files, the skills, and the MCP servers, and no plan sections (planRows returns nothing for NO_PLAN). The picker reads the tree with { workspaceId, repository }, the form #2936 added.
• / skills: useCodingAgents keeps the page agent's repository beside its workspace (both from the page read) and reads the skills through them. The page hands its transcripts those skills (SkillFiles). A skill chip opens its SKILL.md on GitHub, the same way a file mention opens where no File tab stands.
• Dictation keywords come from the agent's repository and title.
• Rows that open a tab: an inbox call's row opens the window's Inbox. Agent and worker rows get their cards (planCards).
• Not in this PR: the side panel on the agent page (opening workers and Changes there). That's C2b, stacked on this one. useAgentComposer is untouched, since #333 keys drafts by agent id.

Based on main (#2936 has merged).

Evidence

Before / after: main vs this branch, the real renderer over a scripted bridge (330's show harness). The red "message box could not be loaded" line is the harness not serving the editor bundle, the same in both.

	Light	Dark
Before (main)
After: reading width
After: ⌘F

• pnpm check:affected: exit 0.
• New tests, each failing on the base sources:
  • desktop-agent-page.test.ts: the page draws the agent pane at the reading width, and ⌘F opens the find bar over its transcript.
  • agent-composer-mentions.test.ts: on a no-plan page, an @ offers the repository's files read through { workspaceId, repository }, and no plan sections.
  • use-coding-agents.test.ts: with no plan open, the skills are read through the workspace and repository the page read named.
  • desktop-agent-page.test.ts: a /release in a message is its chip, and pressing it opens …/blob/HEAD/.agents/skills/release/SKILL.md on GitHub.
• CI is Linux only. It couldn't verify the Mac, and nothing here was run on a Mac.

Invariants

• ./scripts/check.sh green. (CI)
• JSON Schema goldens unchanged.
• Every behavior change has, in this PR, the test that fails without it; no change-detector test added.
• Gateway envelope goldens unchanged.
• No new as type assertion outside the allowlisted files.
• No Effect.runPromise/runSync/runFork outside the runtime edges.
• No new raw async primitives.
• No discarded Effect.
• Test count: +4.
• No hand-rolled file replaced.
• No CLAUDE.md/AGENTS.md sentence names a changed part.
• PRIVACY.md unchanged. Session replay is unaffected: no new attribute carries words.
• Deps unchanged.
• Barrel/door rule unaffected.

Notes

• Blockers or follow-up verification: C2b (the side panel on the agent page: workers and Changes) and C3 (⌘F on every page) stack on this.

🤖 Generated with Claude Code