docs: build notes (default-members, deps policy, docker caching)

CONTRIBUTING.md

@@ -57,6 +57,8 @@ spa/           → React SPA (TanStack Router + shadcn/ui)
 
 Cargo features: `federation` (default; ActivityPub) and `nats`. `sqlite` and `sqlite-federation` remain as deprecated aliases so old build commands keep working.
 
+**Dependencies.** Every external crate is declared once in the root `[workspace.dependencies]`, usually with `default-features = false`; member crates write `foo = { workspace = true, features = [...] }` and enable only the features they use. A new crate goes into both `members` and `default-members` in the root `Cargo.toml` (only `tui` is left out of `default-members`). `cargo machete` should stay clean, and each crate must compile on its own (`cargo check -p <crate> --all-targets`), not just through feature unification in the binaries.
+
 ### Adding a new feature
 
 1. **Domain first** — models in `domain/src/models/`, ports in `domain/src/ports/`, events in `domain/src/events.rs`