chore: add security warning to .env.example to prevent secret leaks (…

[shankumar7]

chore: add security warning to .env.example to prevent secret leaks (fixes #1828)

Summary

Added a strict security warning header to .env.example to ensure contributors do not commit real Supabase keys or GitHub OAuth credentials to version control. This prevents future occurrences of the secret leaks detailed in issue #1828.

Closes #1828

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Type of Change

• Bug fix
• New feature
• Documentation update / Security
• Refactor / code cleanup

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Changes Made

• Added a massive, unmissable security warning to the top of .env.example.
• Reminded contributors to use .env.local for real credentials.

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How to Test

Steps for the reviewer to verify this works:

1. Open .env.example
2. Verify the security warning header is highly visible at the top of the file.

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Screenshots (if UI change)

N/A

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Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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Accessibility Checklist

• Proper keyboard navigation tested
• Responsive UI verified
• Accessibility labels added where needed
  (N/A - This PR does not contain UI changes)

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Additional Notes

Important for Maintainers: As documented in #1828, although the previously leaked keys were removed from this file in commit 73682c3, they remain visible in the repository's Git history (originating in commit a9a5ae1). An administrator must still manually revoke and rotate the Supabase and GitHub OAuth keys to fully resolve the security incident.

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[github-actions]

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