feat: enforce merge-commit policy and markdown wrap convention

- AGENTS.md: require merging PRs with a merge commit (gh pr merge
  --merge), never squash/rebase — the per-PR commit history (review-fix
  and regression-test commits) is intentional record.
- Add scripts/verify-md-wrap.ts: a doc-sync gate that fails on
  hard-wrapped prose paragraphs (one physical line per paragraph), with
  smart exemptions for fenced code, tables, lists, blockquotes, headings,
  HTML comments, hrs, and reference defs. Scope covers README.md,
  docs/**/*.md, packages/*/README.md, plus AGENTS.md / packages/AGENTS.md
  (the files doc-sync did not previously cover). Folded into doc-sync so
  it rides the existing pre-push and CI gates.
- Sync AGENTS.md and docs/development.md doc-sync descriptions and
  command lists to include verify-md-wrap.

package.json

@@ -23,10 +23,11 @@
     "publint": "tsx scripts/publint-all.ts",
     "doc-typecheck": "tsx scripts/doc-typecheck.ts",
     "verify-event-taxonomy": "tsx scripts/verify-event-taxonomy.ts",
+    "verify-md-wrap": "tsx scripts/verify-md-wrap.ts",
     "gen-module-graph": "tsx scripts/gen-module-graph.ts",
     "verify-module-graph": "tsx scripts/gen-module-graph.ts --check",
     "constraints": "tsx scripts/check-workspace-constraints.ts",
-    "doc-sync": "pnpm run doc-typecheck && pnpm run verify-event-taxonomy",
+    "doc-sync": "pnpm run doc-typecheck && pnpm run verify-event-taxonomy && pnpm run verify-md-wrap",
     "hygiene": "pnpm run knip && pnpm run publint && pnpm run constraints",
     "demo:echo": "node --expose-internals --import tsx examples/echo-agent/start.ts",
     "demo:coding": "node --expose-internals --import tsx examples/coding-agent/start.ts",