sync: Add contribution guidelines and fix config file permissions

[agent-smith-k]

Summary

Syncs two changes from upstream:

• Contribution guidelines: Adds CONTRIBUTING.md with instructions for submitting PRs, reporting bugs, and requesting features. Links it from the README table of contents.
• Config file permissions fix: Replaces the write-then-chmod pattern in config and telemetry file writes with atomic writes that create temp files with mode 0600 from the start, then rename into place. Eliminates a brief window where credentials could be read by other local users.

Changes

• New CONTRIBUTING.md with PR guidelines, code style, and security disclosure reference
• README.md updated with Contributing section and TOC link
• src/config.rs: save() now uses atomic_write_restricted instead of fs::write + set_permissions_0600
• src/telemetry.rs: write_instance_id() uses the same atomic write pattern

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