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

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: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/runtime.md

@@ -167,6 +167,49 @@ parsing the log stream:
 | `lastEscalation` | `{ ts, ageMs }` of the most recent Pi auto-escalation. |
 | `reflexPaused` | mirror of the local pause flag (so TUI shows the right state immediately). |
 
+### Skill substrate (Phase 2)
+
+Lives under `runtime/skills/`. A **skill** is a small composable unit of
+survival behaviour with a uniform contract:
+
+```js
+export const skill = {
+  id: "namespace.action",
+  title: "Human label",
+  timeoutMs: 45_000,
+  preconditions(ctx)       -> { ok, code?, detail? }
+  async execute(ctx, args) -> { ok, code, detail, worldDelta }
+  validate?(ctx, result)   -> boolean       // optional
+  recover?(ctx, result)    -> any | null    // optional
+}
+```
+
+Registered skills are dispatched via `runSkill(id, ctx, args)` from
+`runtime/skills/index.js`. The runner enforces the timeout, normalises
+the result shape, runs `validate()` and calls `recover()` on failure
+so the scheduler can act on the hint (e.g. "switch to wander"). Stable
+failure codes the runner itself emits live in `RUNNER_CODES`
+(`unknown_skill`, `precondition_failed`, `timeout`, `threw`,
+`validation_failed`, `done`).
+
+Item/block groups are dynamic: `runtime/skills/groups.js` exposes
+`logs(bot)`, `planks(bot)`, `sticks(bot)`, `beds(bot)`, `foods(bot)`,
+`axes(bot)`, `pickaxes(bot)`, `swords(bot)` — every group is derived
+from `bot.registry`, so a version-sensitive item that doesn't exist on
+the connected server simply doesn't appear in the set and skills
+return `code: "unsupported_version"` instead of crashing.
+
+Reference skills shipped today: `gather.logs`, `survive.eat`,
+`explore.wander`. The reflex loop still calls the older
+`runtime/actions.js` primitives directly — porting more behaviours to
+skills lands in later phases.
+
+Run the contract + groups tests:
+
+```bash
+npm test
+```
+
 ### Optional: prismarine-viewer
 
 Set `VIEWER_PORT=<port>` in `.env` to launch