Add cross-agent contributor skills

[CYX22222003]

Proposed commit message

RepoSense contributors lack shared, repository-specific guidance for
architecture, validation, pull request preparation, review, and docs.

Add portable agent entrypoints and skills for Codex and Claude.

Other information

• Adds canonical skills in .agents/skills and Claude-discovery wrappers
  in .claude/skills, so the guidance has a single maintained source.
• Validation: the skill validator passed for all five canonical skills and
  all five Claude wrappers; git diff --cached --check passed before commit.
• gradlew.bat environmentalChecks was run. It reports missing final
  newlines for the new files in this local Windows setup; the requester
  asked to proceed, and this check is not expected to block CI.
• Application tests were not run because this PR changes only agent guidance
  and skill documentation.