docs: add contribution and security guidance

[davidrotari19]

Summary

Adds public-facing contribution, security, and package metadata for the existing public repository.

Changes

• Add contribution guidelines and Code of Conduct.
• Add security reporting guidance.
• Add license/repository/homepage metadata to crate manifests.
• Harden .gitignore for local config and credential files.
• Remove docs.rs metadata until crates are published.

Verification

• cargo metadata --no-deps --format-version 1
• cargo fmt --all -- --check
• cargo clippy --all --all-targets -- -D warnings
• cargo test --all --all-targets
• cargo test --doc --all

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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