Moves socket control password from the macOS login keychain to a
plain file at ~/Library/Application Support/cmux/socket-control-password.
This eliminates the system keychain prompt that interrupts users on
first launch or after keychain changes.
- Directory created with 0700, file written with 0600 permissions
- One-time migration copies existing keychain password to the file,
deletes the keychain entry, and records a migration version in
UserDefaults so it runs only once
- CLI SocketPasswordResolver also reads from the file path
- Security framework import is now conditional (#if canImport)
- Adds SocketControlPasswordStoreTests covering round-trip, env
priority, path resolution, and migration behavior
Fixes https://github.com/manaflow-ai/cmux/issues/541
163f8572e4