Compact the Debugger step controls and choose the Agent for a live run

Traceability

• Spec ID: studio-debugger-toolbar-and-agent-choice
• Status: Implemented

Intent

The Retained Session Debugger toolbar spent a full row on seven labelled step controls. At the Studio's real width the labels wrapped to two lines ("Previous / Stop", "Step / Into"), pushed the stop conditions past their room, and let the Evidence Cursor marker collide with the last checkbox. A debug transport is the one toolbar where the icons are the vocabulary — a docked workbench renders it as icon targets and keeps the verb in the accessible name and a tooltip.

The New live run dialog had the second problem: it asked for a "Runtime" (Qoder SDK versus ACP) and then silently used whichever ACP Agent the server happened to default to. Studio already discovers a bounded ACP catalog (qodercli, pi, dsh, codex-acp, claude-acp), already publishes it on /api/config as acpAgents, and /api/acp/runs/stream already honours an ?agent=<id> selector. The Debugger was the only launcher that could not name the Agent it was about to run, so a reader could not start a run against Qoder CLI on purpose.

Non-Goals

• Adding a host adapter or a new ACP preset. The catalog stays server-owned and bounded; the browser selects an entry and never supplies a command or argv.
• Changing step semantics, stop conditions, or the Evidence Cursor model.
• Running two Agents from the Debugger; cross-Agent execution stays in Compare.
• Replacing native title tooltips elsewhere in the shell.

Acceptance Scenarios

• AC-1: The seven step controls (Previous Stop, Continue, Next Stop, Step Into, Step Over, Step Out, Previous State) render as icon-only toolbar targets. Each keeps the command verb as its accessible name, so no label text competes for the toolbar row.
• AC-2: Each step control reveals its verb in a tooltip on pointer hover and on keyboard focus. The tooltip is a supplement, never the only carrier of the accessible name.
• AC-3: The toolbar keeps the stop conditions and the Evidence Cursor marker on one row without overlap at wide, compact, and narrow layouts, with no horizontal document overflow. The topbar's session and runtime facts truncate on one line instead of wrapping into the neighbouring column.
• AC-4: New live run asks which Agent answers the prompt. The choices are the local Qoder harness (offered only when a harness is loaded) plus every ACP Agent the server registered in /api/config as acpAgents, named by its own label (for example Qoder CLI).
• AC-5: A registered but unavailable ACP Agent appears as a disabled option marked unavailable, carrying the server's reason as its title. It cannot be selected and cannot start a run.
• AC-6: Starting a run with a selected ACP Agent posts to /api/acp/runs/stream?agent=<id>, and the topbar Agent field names the chosen Agent instead of the server default.
• AC-7: When only one choice exists the selector is hidden, and when no Agent is available the run action stays disabled. A host that predates acpAgents still gets the single default ACP choice.

Plan

• app/run/live-agent-choices.ts (new): the choice model as an exported seam — liveAgentChoices() builds the bounded list from localRunEnabled, the ACP endpoint, and the config catalog; resolveLiveAgentChoice() falls back to the first available entry; liveRunEndpoint() maps a choice to its POST target, appending ?agent=<id> only for a catalog entry.
• app/run/RunView.tsx: make ControlButton icon-only with aria-label plus data-tooltip; replace the runtime state with the choice model, render the Agent selector in the composer, and report the selected Agent's label in the topbar.
• app/App.tsx: pass acpAgents and localRunEnabled from StudioConfig into the Debugger.
• styles/workbench.css: square the step controls to --toolbar-target, add the hover/focus tooltip from existing semantic tokens, stop the stop-condition and Evidence Cursor row from overlapping, truncate the topbar session and runtime meta instead of wrapping them, and rename .live-runtime-select to .live-agent-select with the dialog's own inset.
• i18n (en + zh-CN): replace composer.runtime with composer.agent and add composer.agentUnavailable.

Test & Review Evidence

• packages/harness-studio/test/live-agent-choices.test.ts: the choice list with and without a local harness, catalog absence falling back to one default ACP choice, unavailable entries staying unselectable, the resolver preferring the first available choice, and the endpoint mapping including ?agent= only for a catalog id (AC-4, AC-5, AC-6, AC-7).
• packages/harness-studio/test/browser/acp-debugger.spec.mjs: the fixture now registers a catalog (Fixture ACP, Qoder CLI, unavailable Pi ACP). The composer lists all four choices in order, marks the unavailable preset disabled with the server's reason, and a run started against Qoder CLI reaches the ACP permission gate while the topbar names that Agent (AC-4, AC-5, AC-6).
• packages/harness-studio/test/browser/tool-call.spec.mjs: the retained toolbar exposes seven controls whose accessible names are the verbs, carries no label text in the row, reveals the verb on hover, and reveals the focused control's verb after a keyboard Tab (AC-1, AC-2).
• Verified locally: npx tsc --noEmit clean; 537 studio unit tests pass (the one /api/config failure is a pre-existing acpRuntimeProfile mismatch from unrelated in-flight work, not this change); all 60 Playwright browser tests pass. Toolbar and composer screenshots reviewed at 1440, 1024, and 390 px with documentWidth === innerWidth and no console or page errors (AC-3, AC-7).

Risk

• The verb text is no longer on screen at rest. Mitigated by the accessible name plus a tooltip on both hover and focus, which is the docked-workbench contract for a debug transport; the previous wrapped labels were themselves unreadable.
• Behavior change: an ACP run now names its Agent in the request. Omitting the selector keeps the previous server-default path, so a host without the catalog is unaffected.
• Boundary, not fixed here: the selector can only offer what the server enabled. harness-studio discovers the ACP catalog only when --harness or --experiment is given, and acpAgentEnabled() still requires either an Agent with its own harnessSource or harnessMode: "workspace-default". A zero-configuration Studio therefore shows only the local harness choice even when qodercli is on PATH; enabling discovery there is a separate runtime-scope decision.