fix: self-heal node-pty spawn-helper permissions at runtime

[samwei12]

Summary

Fixes the macOS case where the official terminal plugin installs successfully but new terminal sessions disconnect immediately because node-pty cannot spawn its helper.

Closes #1.

Root cause

CloudCLI intentionally installs plugins with npm install --ignore-scripts, so plugin postinstall hooks are not available for fixing node-pty packaging issues.

On macOS, node-pty's spawn-helper can exist without execute permission. The plugin websocket still comes up, but pty.spawn(...) then fails and the UI drops into an immediate disconnect state.

What changed

• added findModuleRoot() so runtime self-heal resolves the same node-pty copy that the plugin actually loads
• added darwin-only ensureSpawnHelperExecutable() before pty.spawn(...)
• limit the fix to the current architecture's helper
• only add the minimal owner execute bit needed

Verification

• npm install --ignore-scripts
• npm run build
• manually remove execute permission from the current arch spawn-helper
• confirm PTY spawn succeeds after runtime self-heal on macOS arm64