Improve shell completion defaults

[codeforester]

Summary

• Add Bash readline defaults for case-insensitive and clearer completion.
• Add Zsh completion styles for case-insensitive matching and selectable menus.
• Update README and CHANGELOG for the opt-in shell defaults contract.

Validation

• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats --filter "basectl update-profile --defaults enables defaults through profile config" cli/bash/commands/basectl/tests/update-profile.bats
• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats cli/bash/commands/basectl/tests/update-profile.bats
• env -u BASE_HOME BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash ./bin/base-test
• git diff --check

Fixes #869