Add Kani integration tests for skills/kani harnesses

[coderabbitai]

Background

Kani integration tests for the harnesses introduced in skills/kani/references/kani-harness-example.rs and documented in skills/kani/SKILL.md require the Kani toolchain (cargo kani setup) and a Rust nightly compiler. Neither is currently available in this repository's CI environment.

This issue was identified during review of #13 (Add Kani and Verus agent skills for Rust formal verification).

Requested by @leynos.

Work required

• Provision the Kani toolchain in CI (e.g. via a dedicated GitHub Actions job that installs cargo kani and runs cargo kani setup).
• Add integration tests that invoke cargo kani against the harnesses in skills/kani/references/kani-harness-example.rs.
• Gate the Kani CI job appropriately (e.g. on push to main or on PRs touching skills/kani/) to avoid unnecessary overhead.
• Document the CI additions in docs/developers-guide.md under the "Formal verification tooling" section.

References

• PR: Add Kani and Verus agent skills for Rust formal verification #13
• Skill guide: skills/kani/SKILL.md
• Reference harness: skills/kani/references/kani-harness-example.rs
• Kani documentation: https://model-checking.github.io/kani/