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

CHANGELOG.md

@@ -4,6 +4,16 @@ All notable changes to **FORGE** are documented here.
 Format based on [Keep a Changelog](https://keepachangelog.com/); this project uses
 [Semantic Versioning](https://semver.org/).
 
+## [2.3.1] — 2026-08-29
+
+### Changed
+- **PM Insights — collapsible sections.** The **Alert categories** and **Watch list**
+  sections are now accordions: each heading is a button (chevron + a category-count
+  badge, `aria-expanded`/`aria-controls`) that collapses/expands its card grid. Both
+  default to open; the per-card ticket expand and the **Per-PM roll-up** table are
+  unchanged. Adds a `ChevronIcon` (rotates with the section) and a `.grid[hidden]` rule
+  so a collapsed grid actually hides.
+
 ## [2.3.0] — 2026-08-27
 
 ### Added