Add agent-ready repo check

[codeforester]

Summary

• Add basectl repo check --agent-ready to verify the baseline-integrated agent guidance contract.
• Keep repo check --agent-guidance backward compatible while giving --agent-ready a repo init ... --agent-ready repair hint.
• Update help, completions, docs, changelog, and AI command context.

Validation

• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats cli/bash/commands/basectl/tests/repo.bats
• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats cli/bash/commands/basectl/tests/completions.bats
• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats lib/shell/completions/tests/completions.bats
• git diff --check
• bash -n cli/bash/commands/basectl/subcommands/repo.sh
• shellcheck -x cli/bash/commands/basectl/subcommands/repo.sh lib/shell/completions/basectl_completion.sh
• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash HOME=/private/tmp/base-agent-ready-check-smoke-home /Users/rameshhp/work/base-worktrees/1559-add-agent-ready-repo-check/bin/basectl repo init repo --path /private/tmp/base-agent-ready-check-smoke/repo --agent-ready --no-configure
• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash HOME=/private/tmp/base-agent-ready-check-smoke-home /Users/rameshhp/work/base-worktrees/1559-add-agent-ready-repo-check/bin/basectl repo check /private/tmp/base-agent-ready-check-smoke/repo --agent-ready

Fixes #1559