Threshold setup notifications

[codeforester]

Summary

• Send default macOS setup notifications only when setup runs for at least 30 seconds.
• Add --notify as an explicit force option for quick runs while keeping --no-notify and BASE_SETUP_NOTIFY=false as opt-outs.
• Add BASE_SETUP_NOTIFY_MIN_SECONDS to tune the default threshold.
• Warn on macOS when --notify is requested but osascript is unavailable.
• Update README and BATS coverage for thresholded, forced, disabled, dry-run, and missing-osascript behavior.

Validation

• bash -n cli/bash/commands/basectl/subcommands/setup.sh
• bash -n cli/bash/commands/basectl/subcommands/setup_common.sh
• git diff --check
• bats cli/bash/commands/basectl/tests/setup.bats
• bats cli/bash/commands/basectl/tests/basectl.bats