refactor(frontend/library): extract briefing tab sub-components and reorganize panel (#13213)

### Why / What / How

**Why:** `BriefingTabContent` had grown into a single ~260-line file
mixing tab routing, execution lists, agent lists, copilot library
summary, and shared helpers. A mistaken `StripeItem` rename also made
the situation-report UI read like the payments product.

**What:** Extract `BriefingTabContent` sub-components into a local
`components/` folder, reorganize `AgentBriefingPanel` children under
`components/`, keep Sitrep naming consistent, and extract
`buildAutoPilotPrompt` into `SitrepItem/helpers.ts`.

**How:** `BriefingTabContent` is now a thin router;
`CopilotLibrarySummary`, `ExecutionListSection`, `AgentListSection`, and
`EmptyMessage` each live in their own file. Shared tab helpers
(`getAgentStatusLabel`, empty messages, `MAX_VISIBLE`) sit in
`helpers.ts`. `CostsBreakdown` and `StatsGrid` moved under
`AgentBriefingPanel/components/` to match the panel layout pattern.

### Changes 🏗️

- Split `BriefingTabContent` into `BriefingTabContent.tsx` +
`components/{CopilotLibrarySummary,ExecutionListSection,AgentListSection,EmptyMessage}.tsx`
+ `helpers.ts`
- Move `CostsBreakdown`, `StatsGrid`, and briefing tests under
`AgentBriefingPanel/components/`
- Extract `buildAutoPilotPrompt` from `SitrepItem.tsx` into
`SitrepItem/helpers.ts`
- Remove the unused `SitrepList` component
- Remove the animated briefing-panel border
(`AgentBriefingPanel.module.css` + the `styles.glassPanel` class) — an
intentional visual simplification; the same rotating-border effect still
lives in `PulseChips`
- Update imports to use `SitrepItem` (not `StripeItem`) and
`@/app/(platform)/library/types` where relative paths were wrong

### Checklist 📋

#### For code changes:
- [ ] I have clearly listed my changes in the PR description
- [ ] I have made a test plan
- [ ] I have tested my changes according to the test plan:
  - [ ] `pnpm types` — passes
- [ ] `pnpm test:unit --
src/app/(platform)/library/components/AgentBriefingPanel` — passes
(BriefingTabContent + CostsBreakdown helpers)

#### For configuration changes:

- [ ] `.env.default` is updated or already compatible with my changes
- [ ] `docker-compose.yml` is updated or already compatible with my
changes
- [ ] I have included a list of my configuration changes in the PR
description (under **Changes**)

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Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>