refactor(auto-update): make auto-update non-interactive and run in background

[ahmedhesham6]

Description

Refactors the auto-update system from a blocking interactive prompt to a non-interactive background task spawned at startup. This eliminates the y/n prompt that previously blocked the CLI from starting and removes all process::exec()/process::exit() calls from the update success path.

Changes Made

• main.rs: Auto-update now runs via tokio::spawn in the background. Added should_spawn_auto_update() gate that skips update in warden, async, print, or command-modes. Restructured config loading to separate initialization from auto-update gating.
• auto_update.rs: Extracted apply_downloaded_binary_update() from update_binary_atomic() to isolate the file-swap logic for testability. Extracted update_via_brew_with_command() for injectable brew path. Removed process::exec()/exit() — update now returns Ok(()) and prints a restart message. Added update_success_message() for consistent messaging.
• check_update.rs: Removed interactive y/n stdin prompt. Extracted run_auto_update_if_newer() with an injectable async callback for testing.
• warden.rs: Extracted run_stakpak_in_warden_with_path() for testability. Added test verifying STAKPAK_SKIP_WARDEN=1 env propagation to inner process.
• Docs: Updated system prompt and README to say "restart long-running processes" instead of implying auto-restart/re-exec.
• Tests: Unit tests for update gate logic, newer-version skip, brew path injection, atomic binary swap without process exit, and warden env propagation.

Testing

• All tests pass locally (cargo test --workspace)
• No clippy warnings (cargo clippy --all-targets)
• Code is formatted (cargo fmt --check)
• New unit tests cover: update gate conditions, version comparison callback injection, brew update path injection, atomic binary swap lifecycle, warden env propagation

Breaking Changes

None. The update behavior is transparent to users — the only visible change is that updates no longer block startup or prompt for confirmation.