refactor(hm-config): Mark public config structs #[non_exhaustive]

[markovejnovic]

Smell

Config, CloudConfig, and Preferences in crates/hm-config/src/lib.rs are public structs with all-public fields and no #[non_exhaustive]. They are designed to grow over time (config knobs), but every new field is a breaking change for any external struct-literal constructor, and downstream code can build partially-validated configs by hand, bypassing the centralized Default/load_from_paths construction path.

Change

• Added #[non_exhaustive] to Config, CloudConfig, and Preferences.
• Updated the internal save_and_reload_roundtrip test to construct via ..Default::default() / ..CloudConfig::default() instead of fully-enumerated struct literals, so construction funnels through Default (where defaults and future invariants live). Other internal Default impls already centralize field initialization.

This is behavior-preserving: serde derives, defaults, and load/save logic are unchanged.

Type-safety pattern

This applies the #[non_exhaustive] on public config structs pattern (as used in diesel). Marking these structs non-exhaustive makes adding a field a non-breaking change for downstream crates and forces external construction through Default/load, centralizing default values and any future validation invariants. External crates can no longer build partially-valid configs by hand.

CI

Full CI runs on this PR. Locally only cargo check -p hm-config (and cargo test -p hm-config, all green) was run per the scope of this change; the full workspace/TS build was not run locally.