fix(lint): record skill frontmatter values for selector coverage

[swedishborgie]

validateSelectorCoverage checks each user-supplied selector against
lintCollector.allFrontmatterValues, the set of key/value pairs seen across
all discovered markdown files. That map was populated only from
makeMarkdownWalkFunc (tasks/commands/rules); loadSkillEntry recorded the
skill file but never its frontmatter values.

Result: a selector key declared only in SKILL.md frontmatter (e.g. a plugin
key injected by an upstream plugin converter into a skill-only plugin's
SKILL.md) would falsely trigger selector '<k>=<v>' matched no discovered files, even though the matching skill loaded and was included successfully.

Fix: call recordFrontmatterValues(frontmatter.BaseFrontMatter) alongside
the existing recordFile call in loadSkillEntry, matching the order used
in makeMarkdownWalkFunc (record before the MatchesIncludes filter so
coverage reflects every discovered file, not only matched ones).

Adds TestLint_SelectorMatched_FromSkillFrontmatter covering the regression.
No change to runtime (Run()) behavior — MatchesIncludes already parses
the skill frontmatter directly; only Lint()'s coverage check was affected.

[nettapper]

LGTM