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-13-page-chapters.md

@@ -0,0 +1,33 @@
+# Task 13 — Chapter pages ×4
+
+**Agent**: `page-migrator` · **Model**: MiniMax-M3
+**Depends on**: 03, 09 · **Parallel with**: 12, 14, 17 · **Blocks**: 15, 16
+**Worktree**: `.agents/scripts/worktree.sh start 13 page-chapters`
+
+## Goal
+
+`/models/`, `/agents/`, `/skills/`, `/summary/` as Astro routes. One task
+because they share a layout — four near-identical migrations, and doing them
+together is what proves `ChapterLayout` is right.
+
+Blocks 15 and 16 because those pages link here and share the layout.
+
+## Hard constraints
+
+- Four URLs unchanged, **trailing slashes included**
+- `/skills/` has an interactive package explorer (`skills/app.js`,
+  `data-skill-file`, `data-skill-step`) — that becomes an **island**, and it is
+  the only JS across these four pages. The other three ship zero.
+- All `data-*` hooks preserved; `verify.mjs` asserts several
+
+## Done when
+
+- [ ] Four snapshot diffs empty
+- [ ] Three pages ship 0 bytes JS; `/skills/` ships only its island
+- [ ] Screenshots match at four widths
+- [ ] Checklist complete for each page; `npm run gate` green
+
+## Do not
+
+Do not generalise `ChapterLayout` beyond these four. `/rules/` is task 14 and
+has its own interaction model.