chore: remove internal signer Slack skill

[0xhafa]

Remove the internal signer Slack-message drafting skill from the public repository. The grammar check currently requires that file, so remove its skill-specific validations while preserving the shared grammar reference's SHA-256 and version checks.

Validation:

• Reproduced the missing-skill CI failure after deleting the directory.
• pnpm check:signer-grammar, node --check, and git diff --check pass.
• Temporary fixtures confirm the checker still rejects a missing grammar reference, changed bytes, and an incorrect grammar version.
• No remaining references to the removed skill. No contract or grammar-reference changes.

This cleanup targets public main; the private repository's internal skill is unchanged.

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge; no actionable regressions were identified.

Summary

Removes the internal signer Slack-message drafting skill from the public repository.

• Narrows the grammar checker to the retained public grammar reference.
• Preserves missing-reference, SHA-256 integrity, and grammar-version checks.
• Removes the obsolete success message claiming agreement with the deleted generator skill.

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