Add a health-check skill (readiness inspector pattern) to the knowledge plugin

[prorochestvo]

Context

templates/go/CLAUDE.md documents GET /ping (liveness) and GET /health/check
(readiness — "real per-dependency probes") as required routes, and
knowledge:production-stability names "health checks (liveness vs readiness)" as
doctrine. But no skill delivers the concrete implementation contract, so every new
service re-derives the readiness "inspector" pattern from scratch — exactly the kind of
repeated machinery this kit exists to centralize.

Gap

There is no skill that owns the readiness inspector pattern:

• per-dependency probes (DB, repositories, caches, queues, external/AI APIs)
• isolation so one dead dependency never hangs or 500s the whole endpoint
• response shape (per-component status + overall)
• status-code rules (200 only when everything is healthy; a hardcoded OK is unacceptable)
• auth rules for the endpoint
• cost-bounding of paid / rate-limited probes (cache the result and serve its age between refreshes)
• the required tests

production-stability is doctrine-level; the engineer and reviewer agents need the
concrete pattern to implement and to grade against.

Proposal

Add knowledge:health-check (cross-stack — the readiness contract is stack-agnostic
even when the wiring is Go or Dart). Keep production-stability as the doctrine and
health-check as the implementation contract, cross-linked. Wire it into the "context
to load first" lists of engineer.md and reviewer.md for network-facing / new-service
work.

Acceptance criteria

• plugins/knowledge/skills/health-check/SKILL.md exists, description-matched loading.
• Covers: liveness vs readiness split; isolated per-dependency probes; response
  schema; 200-only-when-healthy; endpoint auth; cost bounds for paid probes (cache + age).
• Documents the required tests: each dependency down → that component reported
  unhealthy while the endpoint still responds (no hang, no full 500).
• engineer.md and reviewer.md reference the skill; templates/*/CLAUDE.md point
  to it instead of describing the pattern inline.
• knowledge plugin.json version bumped.