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>

.husky/commit-msg

@@ -0,0 +1,26 @@
+# Conventional commits, matching this repository's existing log
+# (feat:, fix:, docs:, refactor:, chore:, test:, style:, ci:).
+#
+# Deliberately a plain regex rather than a commitlint dependency: this project's
+# thesis is having no unnecessary dependencies, and one regex is legible to
+# every agent that has to satisfy it.
+
+message_file=$1
+first_line=$(head -n1 "$message_file")
+
+# Allow merge and revert commits through untouched.
+case "$first_line" in
+  "Merge "*|"Revert "*) exit 0 ;;
+esac
+
+if ! printf '%s' "$first_line" | grep -qE '^(feat|fix|docs|refactor|chore|test|style|ci|perf)(\([a-z0-9 -]+\))?: .{1,}$'; then
+  echo "commit-msg: subject must be '<type>: <subject>' (lowercase type)." >&2
+  echo "  types: feat fix docs refactor chore test style ci perf" >&2
+  echo "  got:   $first_line" >&2
+  exit 1
+fi
+
+if [ "${#first_line}" -gt 72 ]; then
+  echo "commit-msg: subject is ${#first_line} chars; keep it under 72." >&2
+  exit 1
+fi