feat(protocol): enforce command capability requirements

[msmps]

Summary

• declare exhaustive minimum protocol versions for commands, response payloads, and errors
• probe a daemon through an established legacy-safe command before transmitting future version-gated commands
• reject incompatible operations client-side with actionable restart guidance
• guard daemon request dispatch and substitute legacy-compatible responses when a client cannot decode a newer result
• preserve the single-roundtrip path for every existing command

Verification

• cargo fmt --all -- --check
• cargo clippy --all --benches --tests --examples --all-features -- -D warnings
• cargo test --all (196 passed, 1 intentionally ignored)
• git diff --check

Story

This is Slice A item 6. It establishes the compatibility contract required before the retention ring and logs command introduce the first version-gated wire variants.