Docs cleanup for repository onboarding guidance

[Marius Storhaug (MariusStorhaug)]

Summary

• fix the broken Git Worktrees cross-link in Repository-Defaultsn- add concise copy-paste starters for AGENTS.md, CLAUDE.md, and .github/copilot-instructions.mdn- restore a repo-local CONTRIBUTING.md so the guidance chain is self-contained

Notes

• kept the change limited to onboarding/docs coherence
• aligned the new guidance with the existing pointer-only doc model

[github-actions]

Super-linter summary

Language	Validation result
CHECKOV	Pass ✅
GITHUB_ACTIONS	Pass ✅
GITHUB_ACTIONS_ZIZMOR	Pass ✅
GITLEAKS	Pass ✅
GIT_MERGE_CONFLICT_MARKERS	Pass ✅
HTML	Pass ✅
JAVASCRIPT_ES	Pass ✅
JAVASCRIPT_PRETTIER	Pass ✅
MARKDOWN	Pass ✅
NATURAL_LANGUAGE	Pass ✅
POWERSHELL	Pass ✅
PRE_COMMIT	Pass ✅
SPELL_CODESPELL	Pass ✅
TRIVY	Pass ✅
YAML	Pass ✅

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