Add Modern Web Guidance as the rung-3 lookup for web tasks (#82)

Scoped, optional reference to Modern Web Guidance so the agent can look
up native platform features on web work, filling the gap at rung 3 of
the ladder. Three additive changes, no compact-ruleset surgery:

- skills/ponytail/SKILL.md: "Web tasks: rung 3 lookup" section after the
  ladder. Runtime source only, not byte-compared, so no six-file sync.
- README.md: one "Pairs well with" line, matching the Caveman pattern.
- examples/web-platform-lookup.md: a <dialog closedby> vs Radix
  before/after in the date-picker.md style.

Lookup, not license: MWG suggests, the ladder filters. Absent CLI
changes nothing. No new INVARIANT phrase; rule-copy check stays green.

Co-authored-by: Claude <noreply@anthropic.com>