feat(docs): add docs index generator + initial INDEX.md

scripts/generate-docs-index.mjs walks docs/**/*.md, parses YAML
front-matter (or first H1 fallback) and a Nygard-style ## Status
section, then writes docs/INDEX.md with grouped tables for ADRs,
Specs, Plans (with archive subsection), Patterns, and Other.

Pure Node 20 (no external deps); idempotent — running it twice
produces byte-identical output. Excludes adr/templates/, the ADR
README, INDEX.md itself, and any *_*.md sidecar file.

Wire-up:
- package.json: docs:index → node scripts/generate-docs-index.mjs

Initial run indexed 35 docs across the existing structure; the
generated INDEX.md is committed so the table is reviewable in the
PR before hooking generation into a pre-commit step.

package.json

@@ -15,7 +15,8 @@
     "db:erd": "prisma generate",
     "db:erd:watch": "chokidar \"prisma/schema.prisma\" -c \"npm run db:erd\"",
     "db:insert-milestone": "tsx scripts/insert-milestone.ts",
-    "seed": "prisma db seed"
+    "seed": "prisma db seed",
+    "docs:index": "node scripts/generate-docs-index.mjs"
   },
   "dependencies": {
     "@base-ui/react": "^1.4.1",