feat(mar-882): Ask the chief opens a prefilled composer (redo, no boundary above the heading)

[Thebeatkicks]

Summary

• Redo of feat(mar-882): Ask the chief opens a prefilled composer #349 (reverted as revert: MAR-882 chief prefill (#349) — master shell-smoke proof 1 red #351): "Ask the chief about this agent" now opens the fleet's composer with a prefilled "About <agent title>: " and focus, without wrapping app/page.tsx's default export in Suspense.
• The regression in feat(mar-882): Ask the chief opens a prefilled composer #349 was the Suspense boundary above the fleet page's own <h1>: this app's static export serves the fallback (no heading) as its first paint until useSearchParams resolves, which is exactly what master's post-merge shell-smoke caught (headings: 0 at dash-app://ui/).
• This redo never touches app/page.tsx. ChiefChat already receives the whole fleet as its agents prop, so it resolves its own ?ask=<agent> from window.location.search inside a mount effect instead of taking a prop computed by a useSearchParams call above the heading.
• app/_data/routes.ts (chiefAskHref/CHIEF_ASK_PARAM), lib/copy/chief-chat.ts (describeChiefPrefill), app/_components/composer.tsx (focusSignal), and app/_components/ask.tsx (the two chief-entry links) are reapplied unchanged from feat(mar-882): Ask the chief opens a prefilled composer #349 — none of them depended on how the id reaches ChiefChat.
• New tests/fleet-first-paint.test.tsx source-greps app/page.tsx for Suspense/useSearchParams and renders AgentsPage via renderToStaticMarkup, asserting the <h1> is present — the regression guard for this exact defect.

Test plan

• pnpm typecheck — clean
• pnpm brand:check — passed
• Focused vitest run (12 files, 218 tests) — all passed
• pnpm test (whole suite, 281 files / 5337 tests / 13 skipped) — all passed, no flakes
• pnpm build:renderer + pnpm build:shell — both succeeded
• Local shell-smoke on a scratch store (--user-data-dir/DASH_DATA_DIR both scoped, basename orchestratedash): proof 1 now reads {"headings":1} at dash-app://ui/ (was 0 on master post-feat(mar-882): Ask the chief opens a prefilled composer #349); final tally 85 PASS / 0 FAIL, [smoke] all proofs passed. Scratch runner retired afterward via scripts/retire-scratch-runners.mjs (RETIRED).
• No capture-harness proof that the prefilled text actually lands and focuses on a real screen — see docs/mar-882-handoff.md's "Evidence class" and "What is NOT done".

See docs/mar-882-handoff.md for the full account, including the one open question about a re-render edge case (appliedPrefill remembering a dismissed prefill across an unrelated agents reference change) named under "What is NOT done" item 3.

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