fix: require whole-word skip and a non-empty reason in example annotations

[Ectsang]

parse_annotation() treats any annotation text beginning with skip as a skip:

if text.startswith("skip"):

So skiped, skip_the_check and skipping validation for now each disable validation for that block, silently. _KNOWN_ATTRS catches the same class of typo on the validating path (shema=, directon=), but the skipping path has no guard. A typo that accidentally validates is loud; one that accidentally skips is invisible.

The reason field looks like it's there so every skip stays auditable, but it's optional in the parse — a bare skip returns {"skip": True, "reason": ""}.

Nothing is broken today. All 51 skip annotations in the docs use the documented skip reason="..." form. This is a guard against a future silent loss of coverage, not a live bug.

The fix matches skip as a whole word and returns the existing _error when reason is missing or empty. process_block already handles _error ahead of the skip branch, so nothing else changes.

Tested: all 51 live skip annotations still parse as skips. scripts/test_validate_examples.py goes from 42 to 47 passing, with the same two pre-existing failures (ucp-schema not on PATH).