Rule pack: agent-ux/*

[mike-north]

Rule pack: agent-ux/*

Labels: backlog → P1 when #61 merges (parallelizable with #62–#65 once the engine exists).
Blocked by #61.

Problem

Implement the agent-UX rule pack per docs/specs/lint-engine.md §3.4 — lint for whether the
config will actually trigger and perform well with the model.

Acceptance criteria

1. Rules: agent-ux/description-triggering, agent-ux/frontmatter-hygiene,
   agent-ux/oversized-skill (token-budget threshold configurable via L-D6 config), registered
   with meta per L-D4.
2. Default severity info (L-D7). Heuristics must be deterministic — no model calls.
3. Positive and negative tests per rule with spec-derived expectations; each rule documents its
   false-positive posture in meta.
4. Per-rule docs stubs.
5. pnpm run check + dogfood aipm validate green; changeset (minor, stay 0.x).