Fix wait command returning before editor is ready

[Niaobu]

Summary

Fixes #18 — unityctl wait returned as soon as the WebSocket connected, but the editor could still be blocked on asset imports or safe mode dialogs.

• Bridge now sends editor.ping RPCs after the hello handshake until Unity's main thread responds, proving the editor is truly ready
• Health endpoint includes new editorReady field
• wait command checks editorReady instead of just unityConnected
• Three distinct timeout error messages: no bridge / not connected / connected but not ready
• Intermediate progress: "Unity connected, waiting for editor to be ready..."
• Readiness resets on disconnect, reconnect, and domain reload

Changes

• Protocol: editor.ping command, EditorReady on HealthResult
• Bridge: ProbeEditorReadinessAsync ping loop, IsEditorReady state with && IsUnityConnected guard
• CLI: WaitCommand tracks 3 states with distinct UX per state
• Unity plugin: editor.ping handler
• Tests: 6 new EditorReadinessTests + updated HealthEndpointTests and DtoSerializationTests

Test plan

• dotnet test — 159 tests pass
• Manual test against live Unity Editor: wait shows 3-stage progress, scene list and script eval work immediately after
• Health endpoint returns editorReady: true when editor is responsive
• JSON output includes both unityConnected and editorReady fields