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/templates/pages/interactive.astro

@@ -0,0 +1,45 @@
+---
+// Interactive page template — for pages that genuinely need client-side
+// behaviour (the full guide, the review desk).
+//
+// The page itself is still server-rendered. Only the islands hydrate.
+// If you are copying this for a page that has no interaction, use
+// chapter.astro instead.
+
+import BaseLayout from '../layouts/BaseLayout.astro';
+import Island from '../components/islands/Island.astro';
+import { getCollection } from 'astro:content';
+
+const entries = await getCollection('guide');
+const lang = 'en'; // TODO: wire to the language toggle decision (task 03)
+
+const items = entries.map((entry) => ({
+  id: entry.id,
+  label: entry.data.title[lang],
+  body: entry.data.copy[lang],
+}));
+---
+
+<BaseLayout title="Full field guide" description="TODO">
+  <!-- Static content is server-rendered. No hydration cost. -->
+  <section class="hero">
+    <p class="eyebrow">The full guide</p>
+    <h1>Ship the <em>system.</em></h1>
+  </section>
+
+  <!--
+    client:visible, not client:load — this is below the fold and the page must
+    stay interactive-free until it matters. Justification belongs in the PR:
+    "tab panel requires click-driven state; no server equivalent."
+  -->
+  <Island client:visible items={items} />
+
+  <!-- More static content after the island. Islands are leaves, not wrappers. -->
+  <slot />
+</BaseLayout>
+
+<style>
+  .hero { max-width: 780px; padding: clamp(75px, 12vh, 145px) 0 85px; }
+  h1 { font-size: var(--step-display); line-height: .86; letter-spacing: -.08em; }
+  h1 em { color: var(--blue); font-family: Georgia, serif; font-weight: 400; }
+</style>