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>

.agents/skills/astro-component/SKILL.md

@@ -0,0 +1,68 @@
+---
+name: astro-component
+description: Build one Astro component for the ai-for-dummies site from the project templates. Use when creating a primitive, block, or island, or when splitting existing markup into a component.
+---
+
+# Building an Astro component
+
+## Decide it should exist
+
+Three occurrences, or a name a person says out loud. Two is not enough — see
+[`../../rules/componentization.md`](../../rules/componentization.md).
+
+Then place it:
+
+- `primitives/` — no domain knowledge (Eyebrow, Rule, Callout, CodeBlock)
+- `blocks/` — composed, page-agnostic, takes props (RouteCard, PhasePanel)
+- `islands/` — interactive, justified, a leaf
+
+## Start from a template
+
+```bash
+cp .agents/templates/components/static-block.astro src/components/blocks/RouteCard.astro
+# interactive? use island.astro instead
+```
+
+Templates exist so ten parallel agents produce one house style. Do not start
+from a blank file.
+
+## Write it
+
+```astro
+---
+interface Props {
+  number: string;
+  title: string;
+  summary: string;
+  href: string;
+}
+const { number, title, summary, href } = Astro.props;
+---
+<article class="card">
+  <b>{number}</b>
+  <h2>{title}</h2>
+  <p>{summary}</p>
+  <a href={href}>Open chapter →</a>
+</article>
+
+<style>
+  .card { display: grid; gap: 10px; padding: 22px; background: var(--paper); }
+  b { color: var(--accent); font: var(--font-eyebrow); }
+</style>
+```
+
+Rules that bite most often here:
+
+- **No raw hex.** Tokens only. `check-tokens.mjs` will fail you.
+- **No `client:*`** unless it is genuinely interactive, and say why in the PR.
+- Keep the `gap:1px` over a coloured parent trick where the original used it —
+  it is the house style, not a bug.
+- Preserve every ARIA attribute from the markup you are replacing.
+
+## Prove it
+
+1. Render it at 560 / 800 / 1100 / 1600 px.
+2. Tab to it. Focus ring visible (`outline: 3px solid #a7483f`).
+3. Diff its rendered text against the markup it replaced.
+4. Run [`../../checklists/before-component.md`](../../checklists/before-component.md).
+5. `npm run verify` — green, with no assertion deleted.