feat(workspace): tabbed content surfaces, compact right panel, docked terminal

[enisze]

Summary

Reshapes the workspace layout away from the right-panel "surface" system toward a tabbed model, matching the reference design (terminal always bottom-right, browser/diff/files as tabs, a compact changes/files selection column).

• Browser / diff / file now open as tabs in the top strip beside chats and render in the center; selecting a chat tab restores the timeline + composer.
• New compact RightWorkspacePanel (Changes / Files, w-72) replaces the old wide surface panel. Clicking a changed file or a project file opens it as a center tab. The Run/Tasks dock is always docked at its base.
• Plan renders in the right column beside Changes / Files (not the center), widening the column while shown.
• Terminal is no longer a right-panel surface — it lives only in the bottom-right Tasks dock. rightPanelStore drops the terminal kind + actions, adds showChat, and migrates persisted state (v7 → v8) by stripping terminal surfaces.
• Deletes RightPanelTabs.tsx and the right-panel terminal-surface plumbing / keybindings in ChatView.

Verification

• tsgo typecheck clean
• Lint clean for the changed files (Node 24)
• rightPanelStore unit tests pass (23/23), reworked off the removed terminal surface

Follow-ups / notes

• On narrow/mobile layouts the right column (and thus Changes/Files, Plan, Tasks dock) is hidden — content tabs still work in the center. A mobile home for the dock/selection column is a follow-up.
• Windows window-controls titlebar inset for the new column header isn't reserved yet.
• Not yet verified live in the running app.

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