fix(briefing): render briefing body into content so the model sees it

[mgoldsborough]

Symptom

Calling nb__briefing from chat returns one of "Briefing retrieved from cache.", "Briefing (refreshing in background).", or "Briefing generated." — with no briefing body, including with force_refresh: true. The dashboard renders the briefing fine.

Root cause

nb__briefing's ok() helper put the entire BriefingOutput in structuredContent and only the status note in content (src/tools/core-source.ts). The engine feeds tool results back to the model via extractTextForModel(finalResult.content) (src/engine/engine.ts:843), which reads only content text blocks and never structuredContent (src/engine/content-helpers.ts:100). So a chat caller (the model) saw only the note. The dashboard reads structuredContent directly, so it was unaffected.

nb__briefing is genuinely model-facing — no ai.nimblebrain/internal annotation, and its description tells the model it "Returns a summary of recent activity, upcoming items, and anything needing attention." It must therefore carry the human-readable summary in content, per the platform-tool contract (src/tools/platform/CLAUDE.md §2.1: content = human summary, structuredContent = typed/UI payload).

Longstanding behavior (dates to the briefing tool's origin), not a recent regression.

Fix

Add a pure renderBriefingText(briefing) helper (src/services/briefing-render.ts) that renders greeting + lede + sections grouped by category, and have ok() set content to ${note}\n\n${rendered}. structuredContent is unchanged, so the dashboard is unaffected. Fixes all three branches (cache / stale / generated).

Category and field names anchor on the generator's actual output schema (category ∈ recent/upcoming/attention, field type) from src/tools/platform/schemas/home.ts and src/services/briefing-generator.ts — not the stale names in the inline dashboard script (see note below).

Tests

test/unit/services/briefing-render.test.ts (5 cases): body text reaches the output, greeting/lede included, sections grouped under the correct schema-category labels in order, empty categories omit headings, quiet briefing renders without headings. The category-name test specifically guards the silent-drop failure mode (a renderer keyed on stale names would emit zero sections).

bun run verify:static passes; all briefing unit tests pass.

Follow-up (not in this PR)

The inline dashboard resource script src/tools/core-resources/scripts/home-briefing-inline.ts uses stale category names (needs_attention/coming_up) and a non-existent field (item.sentiment) — it would render zero sections against real output. The primary React dashboard (src/bundles/home/ui/src/App.tsx) uses the correct names and is unaffected. Worth a separate issue/PR to align the inline script (and ideally share this PR's category-order source of truth).