docs(skills): require full-scope completeness bar in gardening-generated issues

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Summary

• The contributor-issue-pipeline gardening skill (.claude/skills/contributor-pipeline-gardening/) generates most of this repo's gittensor:* contributor-available issues. Contributors' AI-harness agents read only the issue text and take the least-effort path unless the text closes off ambiguity.
• Adds an explicit completeness bar: every Requirement must be concrete/testable, every Deliverables checklist item ships together in one PR (no partial credit for a subset), and Expected Outcome must be a falsifiable end-state checkable from the PR diff alone.
• Cross-references the bar from Pass 2 and Pass 3 in SKILL.md, and adds a fixed "ships together" line to the issue body template in reference.md.

Doc-only change under .claude/skills/**; no code/schema/route changes.

Test plan

• Reviewed the diff end-to-end for internal consistency with the existing "Reuse-existing-pattern is mandatory" section
• CI green (doc-only PR, no build/test surface touched)

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

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