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>

.lintstagedrc.json

@@ -0,0 +1,16 @@
+{
+  "*.{js,mjs,ts,astro}": [
+    "prettier --write",
+    "eslint --fix --max-warnings=0"
+  ],
+  "*.css": [
+    "prettier --write",
+    "stylelint --fix --max-warnings=0"
+  ],
+  "*.{astro,css}": [
+    "node .agents/scripts/check-tokens.mjs"
+  ],
+  "*.{json,md,yml,yaml}": [
+    "prettier --write"
+  ]
+}