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/templates/components/grid-group.astro

@@ -0,0 +1,39 @@
+---
+// Grid-group template — for a set of sibling cards separated by hairlines.
+//
+// Note the separator technique: `gap: 1px` over a coloured parent background.
+// That is DELIBERATE house style throughout this site, not a workaround.
+// Do not "fix" it into `border`.
+
+interface Props {
+  label: string;
+  /** Number of columns at the widest breakpoint. */
+  columns?: number;
+}
+
+const { label, columns = 3 } = Astro.props;
+---
+
+<section class="group" aria-label={label}>
+  <div class="grid" style={`--columns: ${columns}`}>
+    <slot />
+  </div>
+</section>
+
+<style>
+  .grid {
+    display: grid;
+    grid-template-columns: repeat(var(--columns), 1fr);
+    gap: 1px;              /* hairline separators, drawn by the parent background */
+    background: var(--line);
+  }
+
+  /* Children paint their own background, which is what makes the 1px show. */
+  .grid > :global(*) {
+    background: var(--paper);
+  }
+
+  @media (max-width: 800px) {
+    .grid { grid-template-columns: 1fr; }
+  }
+</style>