Fix Go runtime version reporting

[TyceHerrman]

Motivation

getRuntimeSummary() uses a generic --version probe for runtime version reporting. Go does not support go --version; it exits with a flag error and prints help text. The supported command is go version.

That means context-mode can correctly detect Go as available, but still report its version as unknown.

Changes

• Update the private runtime version helper to accept explicit version arguments.
• Use go version for Go runtime summary output.
• Keep the existing --version default for other runtimes.
• Use execFileSync with Windows shell compatibility for safer argument handling.
• Add mocked unit coverage to verify Go uses version and other runtimes still use --version.

Verification

• npm test -- tests/runtime.test.ts
• npm run typecheck
• npm run build
• Local sanity:
  • go --version fails with flag provided but not defined: -version
  • go version prints go version go1.26.2 darwin/arm64