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>

.gitignore

@@ -1,3 +1,14 @@
 # Tooling caches, not part of the published site.
 .serena/
 __pycache__/
+
+# Build + toolchain (Astro migration).
+node_modules/
+dist/
+.astro/
+.husky/_/
+
+# Visual-regression working output. Baselines in .agents/snapshots/ ARE tracked;
+# the per-run captures and diffs are not.
+.agents/snapshots/after/
+.agents/snapshots/diff/