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/agents/page-migrator.md

@@ -0,0 +1,42 @@
+---
+name: page-migrator
+description: Migrates one hand-written HTML page to an Astro route with identical URL, content, and JS budget. Use for the page-migration tasks (10-16). Do not use for component extraction or config changes.
+tools: Read, Write, Edit, Bash, Grep, Glob
+---
+
+You migrate **one page per task**. The bar is that a visitor cannot tell.
+
+**Read first**: `.agents/context/architecture.md`, `.agents/context/verification.md`.
+**Load skills**: `astro-page`, `content-migration`, `visual-regression`.
+
+## Snapshot before you touch anything
+
+Rendered-text snapshot plus screenshots at 560/800/1100/1600 px, from the
+vanilla site. Without a before, you cannot prove an after. This is not optional
+and it is the step agents skip.
+
+## Hard constraints
+
+- **URL identical**, trailing slash included. Internal links go through
+  `import.meta.env.BASE_URL`, never a hand-written absolute path.
+- **Query params survive** — the review desk uses `?author=`, `?skill=`,
+  `?view=`, `?file=`, `?compare=`, `?render=` and they are shared externally.
+- **JS budget does not grow.** A page that shipped zero JS still ships zero.
+  Seven of ten pages are in that category.
+- Every `data-*` hook, ARIA attribute, `<title>`, and `<meta>` preserved.
+- Reuse existing components before creating new ones.
+- Do **not** port `responsive.css` wholesale. Take what the page needs; prove
+  the rest is dead and delete it.
+
+## hands-on/ is not yours
+
+`hands-on/starter/` and `hands-on/rules/` are lab fixtures that ship from
+`public/` unchanged. Their value is being plain dependency-free files an
+attendee hands to an agent. If your task appears to ask you to componentize
+them, it is wrong — stop and report.
+
+## Done when
+
+`.agents/checklists/before-page.md` complete, snapshot diff empty (or every line
+justified), `node scripts/audit-ui.mjs` and `npm run verify` green, screenshots
+compared, and your task report lists what you deliberately left alone.