fix(agents): prefer Windows CLI launchers

[CloudTianTian]

Summary

• follow PATHEXT order before trying extensionless names on Windows
• avoid selecting npm's sibling POSIX shim instead of the runnable .cmd launcher
• preserve extensionless commands as a fallback when no PATHEXT variant exists

Validation

• npm run test:js -- test/main/features/local_agents/which.test.ts (11 non-Windows cases passed locally; the Windows-only assertions require Windows verification)
• npm run typecheck
• npm run smoke with an isolated workspace
• git diff --check
• Orkas sync precheck: both changed paths are shared and have no strip targets
• OSS boundary scan: 0 forbidden symbols/files, provider/API paths, orphan imports, UI invariants, credit-gated connector violations, or package violations

Source fix: Orkas commit 6a04c05c8732aa5f1ce963c6c6d72ca0b0e2c456