Story-2029: Brief becomes its own route + bare matter redirects to Brief

The Brief now has its own real URL — /matters/[matterId]/brief — and the bare matter URL (/matters/[matterId]) never renders naked: it redirects to the Brief route. This continues EPIC-2018 (route-per-mode) on top of the fat layout landed in S1 (Story-2019). The Brief body itself is unchanged.

What moved

• New Brief route — the entire Brief body moved verbatim from matters/[matterId]/page.tsx to matters/[matterId]/brief/page.tsx, keeping its existing appGetMatterBrief / FrameStatus / frameVersions server prefetch inside <HydrationBoundary> — so the Brief still lands in the client cache and paints with no refetch flash. It carries the agentSurface key matter.brief (presence key AgentSurfaces.matterBrief, already present in en + hi).
• Bare matter URL is a pure redirect — page.tsx is now a thin server component that redirect()s to the Brief route and exports agentSurface = null (with a // no-agent: reason). The matter's own layout.tsx already owns matterId validation and the shared chrome, so the redirect page does no fetching.

Why the redirect resolves through the surface key, not "./brief"

A literal redirect("./brief") resolves relative to the request URL — against /en/matters/42 the browser would land on /en/matters/brief (the last segment is replaced), which is wrong. Instead the page builds the target from the durable surface key: redirect(localizeHref(locale, buildSurfacePath("matter.brief", { matterId }))). That yields /en/matters/42/brief, keeps the locale prefix, and survives any future route change to matter.brief — and obeys the repo rule to navigate by surface_key, never a string-built raw URL.

Tests

• The Brief body's existing route test moved with it to brief/page.test.tsx (audit log still deferred via after(), first paint doesn't block on the audit fan-out).
• The root page.test.tsx now asserts the redirect target is /en/matters/42/brief.
• tests/unit/matter-detail-route.test.tsx (matter-number resolution M-0001 → M-0013 → M-0120 and the M-ABC not-found case) was repointed to the Brief route, its new home for that resolution + open-audit behaviour.
• navigation.test and the client-tool dispatcher test were updated for the new matter.brief path (/matters/[matterId]/brief).

Deliberately deferred (not this story)

• Routing the placeholder modes (Inbox / Strategy / Chronology / Proof) — S3 (#2035-adjacent).
• Re-homing Record and splitting Filings / Hearings into subpages, and the final deletion of the legacy LiveMatterShell state machine — S4–S7.

Verification

pnpm gen:routes + pnpm check:routes (30 surfaces, fresh), pnpm lint (0 errors), pnpm test (1001 passed), and pnpm build are green. The build output shows both /[locale]/matters/[matterId] and /[locale]/matters/[matterId]/brief.