fix(propose): require shell-safe PR body handling

[pentaxis93]

Summary

• require shell-safe transport for multiline Markdown PR bodies in propose
• explicitly forbid inline double-quoted multiline --body "..." usage
• add safe create/edit examples and remediation for interpolation corruption

Changes

• updated skills/propose/SKILL.md in the Create PR section with mandatory --body-file guidance
• documented acceptable file creation patterns (direct file write, single-quoted heredoc)
• expanded failure policy and corruption modes for shell interpolation corruption and recovery

Issue(s)

Closes #116

Test plan

• inspect skills/propose/SKILL.md and verify:
  • shell-safe PR body transport is required
  • unsafe inline multiline double-quoted body usage is forbidden
  • create/edit safe examples are present
  • failure/corruption remediation references gh pr edit --body-file

[github-actions]

The diff only updates documentation in skills/propose/SKILL.md to require shell-safe PR body handling via --body-file. I did not find a correctness, reliability, or security regression introduced by these changes.

Raw outputs: codex-review.md, codex-review.jsonl