docs(snap): align prover guidance and canary with the approved command alias #4838

Description

Update OpenShell's Snap documentation and release validation to use the approved openshell-prover alias consistently with other installation methods.

Context

The Snap package exposes the standalone policy prover as openshell.prover and already declares openshell-prover as an alias. According to PR #4835, the Snap Store has now granted the automatic alias.

Current documentation still tells Snap users to replace openshell-prover with openshell.prover and warns that the alias may be unavailable. The Snap release canary also invokes only the namespaced command, so it does not exercise the common command name.

Users can work around this by invoking openshell.prover, but that requires installation-specific instructions and leaves the approved alias untested by the canary. Store installations should use the same documented prover command as Homebrew, Debian, RPM, and standalone installations.

Locally built snaps installed directly do not receive the automatic Store alias. Their documentation must retain this distinction and identify openshell.prover as the available command.

PR #4835 implements this cleanup. Its PR Issue Acceptance check currently fails because the community PR has no closing reference to an accepted issue. This issue records the underlying packaging documentation and validation work. A maintainer must record acceptance separately before linking it satisfies that check.

Definition of Done

• Published documentation uses openshell-prover for Snap Store installations and removes obsolete approval warnings.
• Installation documentation explains that directly installed, locally built snaps use openshell.prover without the automatic alias.
• The Snap package description accurately describes the prover command and alias.
• The Snap release canary invokes openshell-prover for its version check and policy boundary check.
• Packaging assertions and contributor troubleshooting guidance match the updated canary behavior.