feat(subagents): add ui-designer builtin preset and template chip

[muzimu217]

What

Adds a fifth builtin subagent, ui-designer, as a "start from template" chip next to Explorer / Code reviewer / Test runner / Fixer.

The delegate takes one interface task with its brief and returns a working, browser-checked implementation — the same kind of self-contained, delegable phase fixer covers (ADR 0089), for the case where the main agent hands off visual work instead of doing it inline at full context cost. The prompt distills a UI-delivery workflow: preserve the project's established design system (existing tokens outrank the delegate's own taste), write a small design contract when none exists (semantic tokens, 4px/8px rhythm, Do/Don't rules), implement the whole interaction including every reachable state, keep motion purposeful with reduced-motion variants, and verify the rendered result in BrowserPreview at desktop and mobile widths before reporting. The report follows the existing <summary>/<changes>/<verification> convention. It is adapted from the author's open-source UI Design Agent Kit, reworked for the delegate's tool set and the no-user-mid-run delegate model.

• Tools: Read, Glob, Grep, BrowserPreview, Bash, Edit, Write at maxTurns: 80 — write-capable like fixer, plus BrowserPreview so it can check its own rendered output. BrowserPreview is already in SUBAGENT_ASSIGNABLE_TOOLS and the host-side tool vocabulary; no host (Rust) change.
• Lockstep: SUBAGENT_PRESETS (shared, the editor's starter values) and BUILTIN_SUBAGENT_DOCUMENTS (agent-runtime, the prompt the sidecar loads) carry identical name/description/tools/maxTurns/body — verified field-by-field by script. The hyphenated id resolves through the editor's catalog map (presetUiDesignerName / presetUiDesignerDesc) with copy in all eight locales.
• Spec pairs updated (en + zh-CN): the settings sheet section (chip row + catalog map), the runtime catalog list / maxTurns table / write-capable wording (correcting the stale "only writable builtin" phrasing), and E2E-142's precondition list.

Deliberately not changed

• The four existing presets' copy, tools and turn caps are untouched; the blank chip stays last.
• decisions-log.md keeps its historical D201 "four builtins" wording — it records the decision as made at the time.
• No new runtime code path: the preset is data for the same parser, merge order and SubagentRun lifecycle as the other builtins (catalog cap 16 leaves headroom at five).

Verification

• Lockstep check: scripted field-by-field comparison of the shared preset vs the builtin document (name, description, tools, maxTurns, body) — all equal.
• pnpm -r test green on the final HEAD: desktop 1463, agent-runtime 363, shared 498, plugin-sdk 224, i18n 23 (8-locale catalog parity), agent-host 34, plugin-devkit 46 — 0 failures. Updated tests: shared presets (five ids; Edit/Write granted only to fixer + ui-designer), agent-runtime builtin parse (five names; mutating = [fixer, ui-designer]; BrowserPreview present), desktop source-contract test (preset id list, tools/maxTurns, body spot-check).
• desktop typecheck, lint, docs:check (77 en/zh pairs) green. No Rust change.
• Live app check: launched the desktop app from this branch and opened Settings → Subagents → New — the UI designer chip renders in the template row, and picking it pre-fills name, description, all seven tools, max turns 80 and the prompt body (verified via CDP with screenshot).

E2E disposition per AGENTS §15 (fork PR):

E2E: NOT RUN
Suite: no listed suite drives the New-subagent template row end to end
Reason: renderer Settings surface; the delegate itself runs the unchanged SubagentRun path shared by all builtins
Alternative validation: unit + source-contract tests above, plus the CDP-driven live check of the template row (chip renders, form pre-fills)
Remaining risk: none beyond the existing builtin delegation path — the preset adds data, not new runtime behavior

[vastsa]

Reviewed the refreshed head 396becc. The ui-designer preset preserves the existing builtin/user precedence and permission model; the landing test correction now covers all five builtins and the designer capability contract. Isolated current-main validation passed Agent Runtime 363/363, Shared 498/498, Desktop 1494/1494, docs locale checks, smoke E2E 18/18 with 2 live-model cases skipped without a key, and subagents E2E 19/19. Required JS, Rust, and docs checks are green.

[muzimu217]

Thank you for the isolated validation and the approval — glad the preset landed cleanly alongside the precedence/permission model. Noted on the verification numbers (Agent Runtime 363/363, Shared 498/498, Desktop 1494/1494, smoke 18/18 + subagents 19/19).

Two market prototype PRs (#285, #290) received your boundary/semantics findings; fixes are starting now and will come back with per-point evidence before re-requesting review.