Feature-gate the Evidence relying-party dependency in registry-breg

Found in the 0.28.0 pre-release review (2026-09-08). Line references are as of main after #926.

`registry-evidence-client` and `registry-evidence-verifier` are the only two dependencies in `crates/registry-breg/Cargo.toml` (lines 227-228) without `optional = true`, so a `default = []` library build still links reqwest and tokio through the client. The always-compiled contract path imports their types: `action_evidence_contracts.rs` and `action_evidence_validation.rs` are ungated `pub mod` in `lib.rs`, and `compiler.rs:174` calls `compile_evidence` on every project compile.

Decision recorded in `products/breg/DECISIONS.md`: 0.28.0 ships the dependency unconditional; the gate is deferred to this ticket.

Proposed change:
- Make both dependencies optional behind an `evidence` feature that `runtime` enables.
- Gate the Evidence-typed contract compile behind the same feature, with a compile-time refusal (clear diagnostic) when a project declares evidence capabilities and the build lacks the feature.
- Collapse `action_evidence.rs`, `action_evidence_client.rs`, `action_evidence_config.rs`, `action_evidence_maintenance.rs` into one `action_evidence/` module tree so the feature has one gate.

Acceptance: `cargo check -p registry-breg` with default features links neither Evidence crate; `bregctl` and `breg` behave unchanged; a test proves the refusal diagnostic.

Start with crates/registry-breg/Cargo.toml lines 227-228 and lib.rs, then inspect compiler.rs:174 and the named action_evidence modules. Run cargo check -p registry-breg with default features first; done means default builds omit both Evidence crates, runtime and bregctl/breg remain unchanged, and a test verifies the refusal diagnostic for evidence capabilities without the feature.

Scritto dal modello di indicizzazione a partire dal testo della issue.