fix(ci): preflight archive candidates before mutation

[nettee]

Summary

• Preflight every selected date-named archive candidate with zest-dev dump <spec> --dry-run.
• Abort with the candidate path and validation error before archive issue lookups, issue creation, or directory removal.
• Add regression coverage proving mixed valid/invalid batches leave GitHub and the working tree unchanged.

Validation

• pnpm test:ci-archive-specs
• pnpm test:ci-version-freshness
• pnpm test:ci-pr-bump
• pnpm test:ci-publish
• pnpm test:local (23 passed, 1 skipped)
• pnpm test:package (24 passed)

Closes #146

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