refactor(prover): remove legacy unused reachability constraints

[elezar]

Description

Audit and remove reachability-model state that is constructed on every prover run but is not consumed by any active query.

The current model still builds credential_has_destructive and filesystem_readable constraints even though no repository caller reads them. binary_can_write is only consumed by can_write_to_endpoint(), which also has no repository callers.

Context

Identified while reviewing the call chain rooted at prove() after #2395.

Relevant code:

• crates/openshell-prover/src/model.rs
• crates/openshell-prover/src/registry.rs
• crates/openshell-prover/src/policy.rs

Because can_write_to_endpoint() is public, confirm whether it is a supported external API before removing the method or the state that supports it.

Definition of Done

• Audit the unused model fields and helper methods against supported public API expectations.
• Remove solver variables, maps, and encoding work that no active or supported query requires.
• Remove associated dead-code allowances.
• Preserve the findings produced by all existing prover fixtures.
• Add or update tests demonstrating that active query behavior is unchanged.