Resize Debugger panes and share streamed messages

Traceability

• Spec ID: debugger-resizable-streaming
• Status: Implemented

Intent

Remove the two pane visibility commands, tighten tree disclosure spacing, resize all three panes through two boundaries, and prove incremental activity display.

Acceptance Scenarios

• AC-1: The title bar has no Execution Tree or Inspector visibility toggles.
• AC-2: Tree disclosure uses one compact target and reduced indentation; no fake expandable live rows with handlers that do nothing.
• AC-3: Both pane boundaries support pointer drag, Left/Right, Home/End, and double-click reset; panes remain bounded as the container changes size. Narrow layouts stack the activity, tree, and inspector with local overflow.
• AC-4: Debugger and Compare render assistant messages through one shared component using the existing shared streaming hook. A partial reply is visible before a second gated chunk and before run completion.
• AC-5: Browser tests cover resizing, incremental text, focus, bounded overflow, and screenshots at wide, compact, and narrow sizes.

Non-goals

New ACP protocols, fabricated reasoning events, or changes to Compare's separate layout redesign. Preserve that concurrent work.

Plan and Tasks

Own layout in ResizableDebuggerPanes; remove collapsed-pane state and CSS. Extract StreamingMessage, keep surface-specific tool presentation, and use a controlled ACP fixture to verify visible deltas before completion.

Test and Review Evidence

• Studio TypeScript emit/app build and final no-emit typecheck passed.
• Six ACP browser flows passed, including both pane boundaries, three widths, default/explicit Agent launch, shared dialog interaction, and Compare.
• The gated two-chunk test confirmed text was visible before the second chunk and completion in both surfaces. The local test used acp-v1-rust with the existing native host and fixture Agent; when that binary is absent the same test explicitly identifies its stdio fallback.
• Streaming projection, run store, and localization: 14 tests passed. Documentation-link graph: 8 passed. Screenshots retained outside shared test-results.
• Review Readiness: user-requested maintenance; no external Story supplied. The worktree also contains concurrent staged/unstaged Studio changes. This scope preserves the separate Compare redesign and reuses its streaming hook. Runtime and UI evidence is from local builds and fixture runs, not a restarted installed application or a measured authenticated production Agent.
• AI involvement: Codex. No commit performed by this task.