ci: standardize workflow naming and add common workflows

[clubanderson]

Changes

• Renamed .yaml → .yml for consistency
• Renamed greeting.yml → greetings.yml where applicable
• Added missing standard workflows: add-help-wanted.yml, stale.yml, greetings.yml, feedback.yml
• Removed OWNERS files from workflows directory (they belong in repo root)

[kubestellar-prow]

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