fix(acp): honor configured terminal shell

[doublecurry]

Summary

• Route ACP shell command fallbacks through the default shell selected in General Settings.
• Load the persisted selection at desktop and server startup, and update live model sessions when it changes.
• Preserve direct execution for structured command-and-argument requests.

Verification

• cargo test --offline --no-default-features --lib shell_config_tests -- --nocapture
• cargo test --offline --no-default-features --lib terminal_shell_setting_persists_and_updates_live_runtime -- --nocapture
• cargo check --offline --no-default-features --bin codeg-server
• cargo clippy --offline --no-default-features --bin codeg-server -- -D warnings