CONTRIBUTING.md + CODE_OF_CONDUCT.md

[prbeegala]

Roadmap section: Operational / repo hygiene
Size: 🟢 Quick win

Problem

Standard public-repo hygiene; not yet present.

Proposed solution

• CONTRIBUTING.md covering: dev setup, the build target, the PR shape, the no-third-party-deps rule, the SemVer policy
• CODE_OF_CONDUCT.md — Contributor Covenant 2.1, standard text

Acceptance

• Both files merged
• Linked from README