Add a renderer-neutral confirmation presenter contract

[ben-ranford]

Outcome

Synthetic persona: Go CLI/TUI integrator. Job: Present consequential action confirmations consistently across human and agent paths.

This is a repository-grounded improvement hypothesis from a synthetic workflow exercise, not validated customer demand.

Evidence

Baseline: 64b0d0d96954 / v1.0.0-rc.2.

• docs/security.md:60-72; action/action.go:597-636; runtime/agent/server.go:40-67
• Existing-surface check (inspected): registry issues bounded single-use confirmations; presentation remains host-owned
• Feature demand is a synthetic workflow hypothesis supported by the documented surface, not validated customer demand.

Bounded scope

The secure confirmation lifecycle exists in action.Registry and agent callback hooks, but there is no shared model for a human confirmation surface.

Acceptance checks

• Presenter receives a redacted confirmation view model and expiry.
• Confirm/cancel routes use the existing registry and cannot bypass authorization.
• Expiry and capacity limits have deterministic fixtures.

Non-goals: No automatic approval. No secret text in presentation payloads.

Compatibility and dependencies

Additive, opt-in API or separately negotiated extension. Preserve existing root v1 signatures, strict wire decoders, canonical encodings, redaction, limits and application action authority. No root dependency or Go-floor increase is assumed.

Dependencies: #53

Milestone plan and shared release prerequisites: #38. Priority: P2. Required verification must preserve existing gates; completing this issue does not itself authorize a release.