refactor: derive_more full + smart-default cleanups

[markovejnovic]

Summary

• Enable derive_more full feature set (was: deref/from/display only)
• Derive IsVariant on all 10 protocol enums — generates is_*() methods
• Replace matches!(event, BuildEvent::BuildEnd { .. }) with event.is_build_end() (3 sites)
• Replace .0.clone() with .to_string() on SnapshotRef newtypes in docker plugin (3 sites)
• Add smart-default 0.7; apply SmartDefault to RegistryConfig (flip auto_discover default to true, simplify 3 call sites to ::default())
• Apply SmartDefault to StepLogWriter (replace manual new() with derive)

Test plan

• cargo check --workspace clean
• cargo check -p hm-fixtures --target wasm32-wasip1 clean
• cargo clippy --workspace -- -D warnings clean
• Unit tests pass (hm-plugin-protocol 13/13, harmont-cli 68/68 excl. pre-existing cloud_run failure)
• Verify no downstream breakage in CI