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>

skills/ponytail/SKILL.md

@@ -40,6 +40,16 @@ Stop at the first rung that holds:
 The ladder is a reflex, not a research project. Two rungs work → take the
 higher one and move on. The first lazy solution that works is the right one.
 
+## Web tasks: rung 3 lookup
+
+On web work, rung 3 is where the laziest win hides: a native element or CSS
+behavior the agent forgot exists. If a web task turns on whether the platform
+covers it (a date input, dialog, popover, view transition, container query),
+and the `modern-web` CLI is available, look it up: `modern-web search "<task>"`,
+then `modern-web retrieve <id>`. It is a lookup, not a license, the answer
+still goes through the ladder. MWG suggests the cutting edge; you keep only the
+rung that holds. Not installed? Skip it, the ladder runs fine without it.
+
 ## Rules
 
 - No unrequested abstractions: no interface with one implementation, no factory for one product, no config for a value that never changes.