fix(checks): suppress name-drift false positives for plugin-namespaced skill names

[protosphinx]

Why

The name-drift check compares a skill's name: frontmatter field literally against the file base name and directory base name. Skills that use the plugin:skill namespace convention (e.g. name: github:search) placed at the canonical path github/search.md always fired a spurious name-drift warning because "github:search" matched neither "search" (fileBase) nor "github" (dirBase) under the existing comparison. This is a real false positive for any author following the namespacing convention described in the README FAQ.

What

• Extended checkNameDrift in src/checks.ts to detect colon-separated plugin-namespaced names. When a name has the shape prefix:local, the check also accepts the canonical split structure: dirBase == prefix AND fileBase == local. The original direct-match path is unchanged, so non-namespaced skills are unaffected.
• Added two new test cases in test/checks.test.ts covering: (1) no drift for a correctly split github/search.md with name: github:search, and (2) drift still fires when the file and directory do not correspond to either part of the namespaced name.

Tests

• Existing 37 tests continue to pass (no behavior change for non-namespaced names).
• New test: does not flag name-drift for a plugin-namespaced skill in canonical split structure - verifies the false positive is gone.
• New test: flags name-drift for a plugin-namespaced skill when dir and file do not match either part - verifies the check still catches genuinely misplaced namespaced skills.

Self-merge gate

• all CI checks pass
• LOC delta < 250 (added + removed)
• no public-API surface change
• no runtime-dependency additions
• no workflow file changes
• tests added or extended

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