chore(sisyphus): notepad scaffold for rule-variants epic

Seeds .sisyphus/notepads/rule-variants/learnings.md with:
  - Ground state (master @ a159299, test counts, existing hooks)
  - Key file anchors verified live (isInCheck, getAllLegalMoves,
    applyMove turn-flip) so the incoming agent doesn't re-grep
  - Command cheat sheet (bun run check, vitest, playwright,
    WS server restart after protocol edits)
  - T3-learned gotchas (tool cap, dev-server hot-reload, module
    load order, pre-commit hook discipline)
  - Empty 'Running progress' section for per-task append-only
    entries

Pairs with .sisyphus/plans/rule-variants-v2.md (execution plan).
The agent picking up this epic reads the plan for the WORK and the
notepad for the CONTEXT.

.sisyphus/notepads/rule-variants/learnings.md

@@ -0,0 +1,72 @@
+# Rule Variants — Execution Notepad
+
+Scaffold for the agent picking up `.sisyphus/plans/rule-variants-v2.md`.
+APPEND to this file (never overwrite) under timestamped headings as
+work progresses.
+
+## Ground state (master @ a159299)
+
+- Baseline: 1417 unit tests, 80/80 Playwright.
+- Preset-flexibility architecture + starting-layouts + modifier-profiles
+  T1/T2/T3 all shipped.
+- Current preset hooks in `packages/chess/src/presets/registry.ts`:
+  - `onActivate`, `onDeactivate`
+  - `onBeforeMove`, `onAfterMove`, `onTurnStart`
+  - `getExtraMoves`, `filterMoves`
+  - `onDamage` (via damage pipeline)
+  - `onCheckGameResult`
+  - `shouldFilterSelfCheck`
+  - `describeMoveEffect`
+  - `pieceAttributes` (declared attrs)
+- This epic adds 4 hooks:
+  - `getRoyalPieces` (A.1)
+  - `filterLegalMoves` (A.2)
+  - `shouldAdvanceTurn` (A.3)
+  - `overridePieceMoves` (A.4)
+- New game fact: `HalfMovesThisTurn` on `GAME_ENTITY` (A.3).
+
+## Key file anchors verified live (April 2026)
+
+| What | Where |
+|---|---|
+| isInCheck hardcoded to "king" | `packages/chess/src/rules/check.ts:83` |
+| findKingPosition | `packages/chess/src/rules/check.ts` (same file) |
+| isCheckmate calls isInCheck | `packages/chess/src/rules/checkmate.ts:80` |
+| isStalemate calls isInCheck | `packages/chess/src/rules/stalemate.ts:92,105` |
+| getAllLegalMoves aggregation | `packages/chess/src/engine.ts:855` |
+| applyMove turn-flip | `packages/chess/src/engine.ts:964` |
+| Per-piece move generation | `packages/chess/src/engine.ts` via `PIECE_TYPE_REGISTRY` |
+| Preset registry | `packages/chess/src/presets/registry.ts` |
+| PIECE_TYPE_REGISTRY | `packages/chess/src/presets/piece-type-registry.ts` |
+
+## Command cheat sheet
+
+- `bun run check` — typecheck + lint + vitest (pre-commit hook gate)
+- `bun run test` — vitest only (NOT `bun test`)
+- `bunx playwright test --reporter=line` — full e2e
+- `bunx playwright test e2e/rule-variants.spec.ts --reporter=line` — new suite
+- `bun run packages/server/src/index.ts` — WS server for MP e2e
+- Fresh server after protocol edits:
+  `tmux kill-session -t ws-server 2>/dev/null; tmux new-session -d -s ws-server -c /home/joey/Projects/rules 'bun run packages/server/src/index.ts'`
+
+## T3-learned gotchas (apply here too)
+
+1. **Tool cap at 200 calls** per delegation. Shard work smaller than
+   you think — 1 preset = 1 delegation is the sweet spot.
+2. **Dev server doesn't hot-reload protocol changes.** Restart the
+   WS server after every `packages/server/src/protocol.ts` edit.
+   (This epic adds no protocol changes, but if any creep in via
+   multiplayer e2e, remember this.)
+3. **Playwright auto-starts the dev-http server** via config; don't
+   start a second one manually. The WS server on :7357 IS separate
+   and needs the tmux restart above.
+4. **`session.insert` is idempotent upsert** — fine to re-seed facts.
+5. **Module-load order matters**: `import "./presets/index.js"` at the
+   top of any consumer that expects all presets to be registered.
+6. **Pre-commit hook is non-negotiable**. Green before every commit.
+   If a commit-message-only retry fails, do a full `bun run check` to
+   see what regressed.
+
+## Running progress
+
+(append entries as you ship tasks, one per task, newest at top)