#23236 Agent chat: the header says where the chat runs (machine · folder · branch)

A started agent chat now says where it runs at the start of its header: the machine, the folder and the branch. Only a new chat's composer names its folder and machine; that location strip goes with the first send. So until now a running chat showed this nowhere unless you opened the Inspector. (Leo's brief for this lane: "surface the same Environment line (machine · folder · branch) at the top of the chat so it's visible without opening the panel.")

• header/EnvironmentLine.tsx (new) reads the same snapshot as the Inspector's Environment section and uses its machineOf, so the two never disagree. That's also where the machine-identity lane's colors and icons will plug in. While the Inspector is on, clicking the line opens the Inspector at Environment; otherwise the line is a plain note. The tooltip is the folder's full path.
• Placement:
  • In a header with its own row, the line sits in the status slot. It never sizes the slot: each name ends in an ellipsis, so the tools stay put.
  • In a header that shares the pane's tab row (on by default), the slot is under the tabs, so a compact version capped at 220px leads the tools, the way the connection warning already does there.
  • I didn't add a new bar above the chat, because the tab row exists to remove the top bar.
• A connection problem's label takes the line's place in both placements (ChatHeaderStatus, ChatHeaderInlineStatus).
• No new strings: the label reuses rail.environment.

Tests

• webviews/test/chat-environment-line.test.tsx covers the slot line and its click, a warning replacing the line and the line coming back, the compact tab-row line, an SSH chat naming its host, the note without the Inspector, and no line before the session says where it runs.
• The gallery's agent-pane.header entry has new long-branch and tab-row states, and covers EnvironmentLine.

Observed / Not run

• Observed: the TypeScript check is clean for every touched file (the only errors were from the checkout's stale ignored generated/ strings). The pre-push gate is clean.
• Not run: bun tests, the gallery and a live pane, because this host (cosmic-gray) runs no tests. CI's webview shards and the gallery advisory run cover them.

Changelog

Added: An agent chat's header names the machine, folder and branch the chat runs in.

🤖 Generated with Claude Code

Agent session: cosmic-gray

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Summary by cubic

A started agent chat's header now names where it runs (machine · folder · branch), so you can tell chats apart without opening the Inspector.

• The new EnvironmentLine reads the same snapshot as the Inspector's Environment section, so the two never disagree.
• It sits in the header's status slot (or leads the tools in a tab-row header), with long names ellipsized so the tools stay put.
• Clicking it opens the Inspector at Environment; a connection problem's label replaces it in both placements.
• The line only renders once the session reports where it runs.