Improve issue and epic planning guidance

[vitormattos]

Summary

Improve the create-issues-and-epics skill to cover progressive epic decomposition, native GitHub relationship ownership, UX/prototyping expectations, product documentation as part of done, external GitHub reference hygiene, and AI-specific planning boundaries.

Why

Recent LibreSign planning exposed recurring gaps that should be handled consistently by the reusable skill rather than rediscovered per feature.

Main changes

• Treat native GitHub parent/sub-issue and blocking relationships as authoritative.
• Add progressive elaboration and downstream epic handoff guidance.
• Require UX research/prototyping for new or material interaction changes.
• Treat user/admin documentation as a feature deliverable rather than an implementation diary.
• Avoid noisy cross-references to external GitHub issues/PRs used only as research.
• Add a focused AI planning reference covering human authority, trust boundaries, policy, provider ownership, UX, testing, and deferred platform metadata.

Validation

The commit was created with DCO and cryptographic signing. Local repository checks could not be run because the execution environment cannot resolve GitHub for cloning; CI should run the repository's normal unittest and reuse lint checks.