Define the fleet MSRV policy at Rust 1.89

[pierrick-fonquerne]

Summary

• define Rust 1.89 as the August 2026 fleet baseline
• document the twice-yearly review cadence, release contract, repository exceptions, and dependency decisions
• pin the shared PR-review tool and its CI to Rust 1.89

Why

The fleet previously encoded Rust 1.88 independently in each repository without a single policy for review cadence, exceptions, dependency resolution, or container actions.

Validation

• cargo +1.89.0 fmt --all -- --check
• cargo +1.89.0 clippy --workspace --all-features --all-targets -- -D warnings
• cargo +1.89.0 test --workspace --all-features
• cargo +1.89.0 check --workspace --all-features --all-targets
• cargo audit

Closes #28