docs: add .agents workspace and the Astro refactor plan

Adds the agent-facing workspace and a 20-task plan for migrating the site
to Astro. Nothing here implements the refactor; these are briefs, rules and
templates that the task agents read.

- .agents/ holds context, rules, checklists, skills, specialist agents,
  component/page/config templates and gate scripts. It is vendor-neutral so
  MiniMax, Gemini and Codex can all read it; CLAUDE.md just points at
  AGENTS.md.
- .husky/ plus .lintstagedrc.json wire the three gate tiers. gate.sh locks on
  the shared git-common-dir so parallel worktrees serialise, and guards the
  assertion count in scripts/verify.mjs against a coverage drop.
- plans/astro-refactor/ carries the phase graph, per-task briefs and the
  model-routing recommendation.

These files must be tracked before fanning out: a worktree only checks out
tracked files, so an untracked plan is invisible to every agent working in one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

plans/astro-refactor/task-12-page-landing.md

@@ -0,0 +1,30 @@
+# Task 12 — Landing page
+
+**Agent**: `page-migrator` · **Model**: MiniMax-M3
+**Depends on**: 03, 08 · **Parallel with**: 13, 14, 17
+**Worktree**: `.agents/scripts/worktree.sh start 12 page-landing`
+
+## Goal
+
+`index.html` → `src/pages/index.astro`. Smallest page migration; sets the
+pattern the other page tasks follow.
+
+## Steps
+
+Snapshot first (task 03 baseline exists — diff against it). Then compose from
+`RouteCard` + `GridGroup`, move copy to the `chapters` collection, port
+`landing.css` into component styles.
+
+## Hard constraints
+
+- URL stays `/ai-for-dummies/` exactly
+- **Zero JS** — this page has none today
+- `<title>`, `<meta name="description">`, viewport preserved
+- Internal links via `import.meta.env.BASE_URL`, never hand-written absolute
+
+## Done when
+
+- [ ] `diff .agents/snapshots/index.txt <(snapshot of dist)` empty
+- [ ] Screenshots match at four widths
+- [ ] Built page ships **0 bytes** of JS
+- [ ] `.agents/checklists/before-page.md` complete; `npm run gate` green