Host app Not Responding on macOS 26.5 / Xcode 26.5 — Settings stuck loading, chat and menu bar unresponsive (0.42 / 0.50 / 0.51)

## Describe the bug

The **GitHub Copilot for Xcode host app** becomes unresponsive shortly after launch. In Activity Monitor it shows as **Not Responding** (red).

While the app window can open, the UI is effectively dead:
- **Settings → GitHub Account** stays on "loading…" indefinitely
- **Settings → Permissions** stays on "loading…" indefinitely
- **Chat window** does not open
- **Menu bar icon** does not respond to clicks

The Extension Service and Communication Bridge processes start and sometimes connect (feature flags / policy updates appear in logs), but the host app UI never becomes functional. The failure has persisted for several weeks across multiple Copilot versions and after a full clean reinstall.

## Versions

- Copilot for Xcode: **0.51.0** (also reproduced on **0.50.0** and **0.42.0**)
- Xcode: **26.5** (Build 17F42)
- macOS: **26.5.2** (Build 25F84)
- Hardware: **MacBook Pro, Apple M5 Pro, 24 GB RAM**

## Steps to reproduce

1. Install GitHub Copilot for Xcode in `/Applications/GitHub Copilot for Xcode.app`
2. Grant required permissions:
- Accessibility
- App Background Activity (Login Items)
- Xcode Source Editor Extension
3. Launch Xcode and open a Swift file
4. Launch **GitHub Copilot for Xcode** (host app)
5. Observe in Activity Monitor: host app goes **Not Responding** within ~30–60 seconds
6. Open Settings → Account / Permissions → stuck on loading
7. Try opening Chat or clicking the menu bar icon → no response

## Expected behavior

Settings should load account status and permissions. Chat and menu bar should be usable.

## Actual behavior

Host app freezes. Settings spin forever. Chat and menu bar are unresponsive.

Representative errors (repeated across sessions):

[error] [Client] Failed to update feature flags: Waiting for service to connect to the communication bridge.
[info] [Client] XPCService interrupted
[error] [UI] Failed to reload status. Connection to extension service error: Couldn't communicate with a helper application.
[error] [Client] AgentModeDropdown: Failed to load modes: Connection to extension service error: Couldn't communicate with a helper application.
[error] [Service] Failed to connect to bridge (1 consecutive failures): The operation couldn't be completed. (Swift.CancellationError error 1.)
[error] [Service] AXObserver: Accessibility API disabled, will try again later

On some launches the bridge appears to connect successfully:

[info] [CommunicationBridge] Service app found: ... Optional("com.github.CopilotForXcode.ExtensionService")
[info] [CommunicationBridge] Service app is still valid
[info] [Client] Feature flags updated: agentMode=true, byok=true, mcp=true, editorPreview=true
[info] [Client] Copilot policy updated: customAgent=true, mcp=true, subagent=true

…but the host app UI remains frozen regardless.

## Troubleshooting already tried

- [x] App installed in `/Applications` (not a custom path)
- [x] Official uninstall script + delete `~/.config/github-copilot/` + reinstall from DMG
- [x] Tested **0.42.0**, **0.50.0**, and **0.51.0** — same behavior on all three
- [x] Full Mac reboot after reinstall / upgrade
- [x] Force-quit all Copilot processes and relaunch
- [x] Verified Accessibility, Background Activity, and Xcode Source Editor Extension permissions
- [x] Disabled **Proxy strict SSL** (`GitHubCopilotUseStrictSSL = false`)
- [x] Reset TCC permissions for Copilot bundle IDs and re-granted manually

After clean reinstalls, `~/.config/github-copilot/auth.db` is empty (0 bytes), so no OAuth session persists — but the UI freeze happens even before sign-in can complete.

## Related issues

- #731 — Settings stuck on "account loading" (similar UI symptom; workaround was moving app to `/Applications`, which does not apply here)
- #746 — Host app Not Responding on versions > 0.42 (similar symptom, but on **macOS 15.7.3 / Intel**; downgrading to 0.42 worked for that reporter — it does **not** work for me)

## Additional context

- Issue started several weeks ago and has been reproducible consistently since.
- Antivirus (Avast) and VPN clients are installed but were previously used without issues; Copilot worked fine before this regression.
- This appears to be specific to the **macOS 26.5 + Xcode 26.5** environment, since downgrading Copilot and full reinstalls do not help.

Start by reproducing the freeze in the GitHub Copilot for Xcode host app on macOS 26.5 with Xcode 26.5, then inspect the reported Client, UI, CommunicationBridge, and Extension Service connection errors. Done means Settings account and permissions load, Chat and the menu bar respond, and the host app no longer becomes Not Responding.

索引モデルが issue の本文から書いたものです。