feat(runtime): skill substrate + dynamic groups + reference skills (Phase 2) (#14)

Phase 2 of plans/autonomous-survival-bot-prd.md. Establishes the
composable skill contract from PRD §5.2 and ports three reference
skills so future phases can layer survival behaviour on top instead of
adding more ad-hoc branches to reflex.js.

New: runtime/skills/
- index.js: skill registry + runSkill(id, ctx, args) wrapper. Enforces
  preconditions, hard timeout, normalises {ok, code, detail, worldDelta}
  on every result, runs validate() and calls recover() on failure.
  Stable failure codes live in RUNNER_CODES (unknown_skill,
  precondition_failed, timeout, threw, validation_failed, done).
- groups.js: registry-derived item/block sets — logs/planks/sticks/beds
  derived by suffix; foods intersects a curated allowlist with the live
  bot.registry; axes/pickaxes/swords scoped to whatever the connected
  server's item table actually ships. Empty set instead of throwing on
  missing registry, so skills can emit code:"unsupported_version".
- chop-logs.js: gather.logs reference skill (wraps chopNearestTree).
- eat.js: survive.eat (wraps eatBestFood, preconditions check carrying
  edible food from the registry-derived set).
- wander.js: explore.wander (wraps wander).
- contract.test.js + groups.test.js: 14 tests covering precondition
  gating, timeout firing recover(), execute exceptions, validate
  flipping ok→false, dynamic group filtering across mock registries.

package.json: `npm test` runs the new contract + groups suites.
docs/runtime.md: documents the skill contract, runner, dynamic groups
and the reference skills.

Reflex.js still calls actions.js directly — wiring the scheduler to
runSkill() lands in later phases when the survival curriculum kicks in.

Co-authored-by: Yuriy Mayatnikov <mayatnikov@me.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -15,7 +15,8 @@
     "bot:bare": "node runtime/bot.js",
     "tui": "tsx tui/tui.tsx",
     "propose:apply": "node scripts/propose-apply.js",
-    "stop": "bash scripts/stop.sh"
+    "stop": "bash scripts/stop.sh",
+    "test": "node --test runtime/skills/contract.test.js runtime/skills/groups.test.js"
   },
   "dependencies": {
     "dotenv": "^16.4.5",