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/rules/theming.md

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+# Rule: theming
+
+Binding for every colour, font size, spacing value, and breakpoint.
+
+**Read [`../context/design-system.md`](../context/design-system.md) first.** The
+current CSS has three drifting palettes and a broken `@font-face`. This rule
+describes the target; that file describes what you are migrating from.
+
+## One token layer
+
+Every value comes from `src/styles/tokens.css`. If a component needs a value
+that is not a token, either it is a genuine one-off (justify it in a comment) or
+the token layer is missing something (add it there, not inline).
+
+```css
+/* forbidden */
+color: #172f42;
+color: rgba(23,47,66,.6);
+
+/* required */
+color: var(--ink);
+color: var(--muted);
+```
+
+No raw hex outside `tokens.css`. `.agents/scripts/check-tokens.mjs` enforces it;
+wire it into `npm run verify`.
+
+## Semantic names, not literal ones
+
+`--ink`, `--paper`, `--muted`, `--line`, `--accent`, `--gold`, `--blue`,
+`--deep` are the existing vocabulary. Keep it — it is already semantic and the
+team reads it fluently. Do not rename to `--color-neutral-900`.
+
+If a genuine second surface is needed, extend semantically
+(`--surface-lab`, `--ink-inverse`), never numerically.
+
+## Type scale
+
+Replace the 14 ad-hoc `clamp()` triples with named steps:
+
+```css
+--step-display: clamp(56px, 9vw, 126px);   /* h1 */
+--step-6: clamp(36px, 5vw, 65px);          /* section h2 */
+--step-5: clamp(24px, 3vw, 38px);          /* sub-head */
+--step-4: clamp(22px, 3vw, 36px);          /* pull-quote */
+--step-1: 15px;                            /* body */
+--step-0: 11px;                            /* eyebrow / label */
+```
+
+The eyebrow treatment (`10–11px` monospace, `letter-spacing:.08–.1em`,
+uppercase) is a signature of this design. Make it one class, not fifteen
+repetitions.
+
+## Breakpoints
+
+Five named widths replace the current sixteen:
+
+```css
+--bp-sm: 560px;  --bp-md: 800px;  --bp-lg: 1100px;
+--bp-xl: 1600px; --bp-2xl: 2200px;
+```
+
+When collapsing a component's old breakpoint onto a named one, screenshot at the
+**old** value. That is where the regression will be.
+
+## Preserve the house style
+
+- Flat colour blocks, hairline `1px` rules, near-zero border-radius.
+- Tight negative tracking on display type (`-.06em` … `-.08em`).
+- Grid separators built as `gap:1px` over a coloured parent background. This is
+  deliberate. Do not "fix" it into `border`.
+- `Georgia, serif` for emphasis spans (`h1 em`). It renders today; keep it.
+
+## Fonts
+
+Do not add a webfont without an explicit decision recorded in the task. The
+intended Manrope/DM Mono has never rendered; introducing it is a visual redesign,
+not a refactor. Default: match what renders today.