validatePinnedGitHubRelease canonical SemVer

Trap: semver.valid(version) !== null looks like strict validation, but SemVer normalizes v1.2.3, allowing noncanonical Toolkit tags such as cli@v1.2.3 and direct pinned probes before rejection. Fix: require semver.valid(version) === version before network access; Toolkit accepts canonical cli@VERSION, while legacy normalization remains confined to its adapter. Stable metadata must also reject drafts and GitHub/SemVer prereleases.