Make BATS an opt-in dev dependency

[codeforester]

Summary

• Treat BATS as a developer/test dependency instead of a required runtime dependency
• Add basectl setup --dev to install developer dependencies such as bats-core
• Add basectl check --dev to include developer dependency checks
• Stop installing BATS during default basectl setup
• Stop failing default basectl check when BATS is missing
• Update JSON check output so BATS appears only in --dev mode
• Update README, TODO, AI context, and BATS coverage

Behavior

Default runtime setup/check:

basectl setup
basectl check
basectl check --format json

Developer setup/check:

basectl setup --dev
basectl check --dev
basectl check --dev --format json

Testing

bash -n cli/bash/commands/basectl/subcommands/setup.sh cli/bash/commands/basectl/subcommands/check.sh cli/bash/commands/basectl/subcommands/setup_common.sh
bats cli/bash/commands/basectl/tests/setup.bats
bats cli/bash/commands/basectl/tests/basectl.bats
bin/basectl check --format json
bin/basectl check --dev --format json