feat: offer Neva CLI upgrade from VS Code

[emil14]

Summary

• replace the internal “started directly” warning with a user-facing explanation and an Upgrade Neva action
• run neva upgrade in a visible VS Code terminal; Run remains explicitly gated on restarting with a current CLI
• bound the legacy CLI probe so an unresponsive old CLI cannot block extension activation
• register the upgrade command in the Extension Host contract and repair the macOS VS Code executable path used by the test harness

Verification

• npm run build passes locally
• the Extension Host contract starts on VS Code 1.131.0 and confirms the new command registration; the local runner then hangs during its pre-existing host teardown, so CI is the authoritative full integration result.