[WIP] Fix post_version hook aborting semver set run on missing file

[Copilot]

• Inspect the post-hook flow, output helpers, tests, and recent workflow run status
• Add recoverable missing-file warning handling for post-version hooks while keeping other hook errors fail-fast
• Surface hook warnings via GITHUB_OUTPUT and write warnings to stderr
• Add focused tests for warning collection/continuation behavior and output emission
• Run targeted validation, then full required Deno checks and manual CLI verification
• Run secret scan, code review, and CodeQL before finalizing

• Fixes post_version hook aborts entire 'semver set' run when hook target file is missing #171

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[justinmchase]

Already imlemented.