Add the Destructive Command Guard binary service and runner

[navedmerchant]

Parent

Part of #1049. This is the second independently reviewable slice of the Destructive Command Guard work.

Dependency

Depends on the shared managed-binary infrastructure issue created as the first slice.

Problem

The extension needs a small, testable service boundary for selecting, installing, verifying, and invoking the pinned DCG binary. Command-execution policy should not own platform/archive/process details.

Scope

• Define the pinned DCG version and supported platform/architecture archive metadata.
• Install and verify DCG through the shared managed-binary infrastructure.
• Cache/deduplicate installation within the extension process while allowing retries after failure.
• Run DCG with a command and working directory, parse its structured result, and map it to a typed allow/deny result.
• Treat unsupported platforms, malformed output, process failures, and timeouts conservatively.
• Add focused manager and runner tests.

Out of scope

• Persisted settings or settings UI.
• Auto-approval policy and command tool integration.
• Chat rendering for denied commands.

Acceptance criteria

• Supported platforms resolve pinned archives with checksums.
• Installation uses the shared managed-binary lifecycle.
• Concurrent callers share one installation attempt and failures can be retried.
• Runner output is parsed into a typed allow/deny result with optional reason/rule metadata.
• Runner failures and invalid output do not accidentally classify a command as safe.
• Focused DCG service tests pass.
• The stacked branch type-checks/builds independently.

Stack

Base: the managed-binary PR. The settings issue will build on this issue.