Handler ABI v2 trial status is stated in prose only

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

`handler.abi` is one required string enum with two values (`crates/registry-breg/src/contract.rs`, `ACTION_HANDLER_ABI_V1` and `ACTION_HANDLER_ABI_V2`); `registry.action-handler/v2` is the value that unlocks `evidence::resolve`. Its trial status appears only in `products/breg/immediate-actions.md:628-716` and `products/breg/EVIDENCE.md:161`. It is invisible in the generated JSON schema, in CLI help and on the docs site, and nothing stops frozen contract material from depending on it.

Proposed: state the trial status in the schema description of the v2 enum value (so the generated configuration reference shows it) and in `bregctl explain` output, or drop the word everywhere. Recommendation: keep it and make it visible.

Start with the enum definitions in crates/registry-breg/src/contract.rs, then inspect products/breg/immediate-actions.md, products/breg/EVIDENCE.md, the generated JSON schema, and bregctl explain output. Confirm how the schema description and CLI text are produced. Done means the v2 trial status is visible consistently in the configuration reference and explain output.

Written by the indexing model from the issue text.