fix(desktop): keep member runtime status off the UI thread

[matt2e]

Summary

• skip managed-agent runtime discovery when the members sidebar has no local managed bots
• run runtime listing disk, process, and mutex work on Tauri’s blocking pool
• preserve local managed-bot status and Start/Stop behavior with positive and negative E2E coverage

Opening Add people in a human-only channel could invoke synchronous native runtime discovery before the sidebar painted, leaving the macOS app beachballed. Human invites do not depend on that data.

Related issue

Related overlap: #4851 is a broader managed-agent lifecycle change that includes a similar native offload. This draft is intentionally limited to the sidebar critical path and adds the human-only query gate.

Testing

• just ci
• just desktop-check
• just desktop-test (5,241 passed)
• just desktop-tauri-fmt-check
• just desktop-tauri-clippy
• just desktop-tauri-test (2,702 passed; 18 ignored)
• focused Playwright: human-only sidebar skips runtime discovery
• focused Playwright: local managed bot retains status and Stop/Start controls

No visual styling changed, so screenshots are not applicable.