feat(insights): collapsible Alert categories + Watch list accordions

The PM Insights "Alert categories" and "Watch list" sections are now
accordions: each heading is a button (chevron + category-count badge,
aria-expanded/aria-controls) that collapses/expands its card grid. Both
default open; the per-card ticket expand and the Per-PM roll-up table
are unchanged.

- Section accordion component + sections state in Insights/index.tsx
- ChevronIcon that rotates with the section's open state
- .grid[hidden] rule so a collapsed grid actually hides (beats .grid
  on specificity + source order)
- bump 2.3.0 -> 2.3.1 (root + client), CHANGELOG [2.3.1]
- engineer artifact + INDEX line; verifier-log PASS entry

verifier PASS (verifier-log 2026-08-29 12:07): reproduced tsc + vite
build; confirmed the [hidden] collapse in the compiled CSS; no
per-card regression; no new dependency.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VEsaHQx8cXr1hFrKU42UK6

claude_artifacts/INDEX.md

@@ -1,4 +1,5 @@
 # Artifact index
+- [engineer-20260829-120746](engineer-20260829-120746.md) — PM Insights: "Alert categories" + "Watch list" now collapsible accordions (new `Section`, `ChevronIcon`, `.grid[hidden]` collapse, `aria-expanded`/`aria-controls`). v2.3.1. verifier PASS.
 - [engineer-20260827-190753](engineer-20260827-190753.md) — Jira statusDurations+movements + chart #17 (Active tab), /api/analytics/jira-durations, seed-enriched from board_state. v2.3.0.
 - [engineer-20260827-185932](engineer-20260827-185932.md) — Extension Jira sync (SNOW→Jira, attach-only /api/sync/jira, custom-domain + PAT). v2.2.0. Live-verified attach-only.