Add contributor guide, privacy-safe issue templates, and PR checklist

[ohad6k]

Contributors can find open work, but the repository does not yet explain the privacy boundary or what evidence a safe Ditto contribution needs.

Add a small contributor baseline that makes useful participation easier without inviting anyone to upload private logs or generated profiles.

Acceptance criteria

• Add CONTRIBUTING.md with local setup, the full test command, focused-test examples, and the expected verification report.
• State clearly that raw sessions, generated profiles, receipts, secrets, and local paths must never be committed or attached to an issue.
• Add issue templates for bugs, source-adapter requests, and documentation/feedback.
• Make templates ask for minimal redacted reproduction details and environment/version evidence.
• Add a pull request template with scope, tests run, privacy review, and documentation-impact checks.
• Add a standard code of conduct and a clear reporting contact.
• Preserve the existing issue forms and repository behavior if any appear before implementation.
• Verify GitHub recognizes the files and that the repository contains no private test artifact.

This issue does not add telemetry, contributor marketing requirements, or any request to star or promote Ditto.