ci: add Rust quality job (fmt, clippy, test)

[rubenhensen]

Summary

• Adds a quality job to .github/workflows/ci.yml that runs cargo fmt --all -- --check, cargo clippy --all-targets -- -D warnings, and cargo test --all-targets on every PR and on push to main.
• Runs in parallel to the existing release-plz / Docker pipeline; does not block it.
• Implements step 3 from CI does not run cargo test, clippy, or fmt — Rust quality regressions can land silently #126 — the patch dobby-coder verified locally but could not push due to missing workflows: write permission on the GitHub App.

Local verification on main@a058c3d + this change

Step	Result
cargo fmt --all -- --check	clean
cargo clippy --all-targets -- -D warnings	clean
cargo test --all-targets	30 passed, 0 failed

Test plan

• CI quality job runs and passes on this PR
• Existing release-plz / Docker pipeline still runs unchanged on push to main

Closes #126.