Implement basectl onboard

[codeforester]

Summary

Implements the v1 basectl onboard checklist flow from docs/basectl-onboard.md.

• adds a Bash onboard subcommand with --dry-run, prompted, --yes, --dev, --no-profile, -v, and help flows
• orchestrates existing check, setup, update-profile, doctor, and projects list commands without duplicating their internals
• wires onboard into top-level basectl help and dispatch
• adds BATS coverage for help, dry-run, declined setup, accepted setup, --yes, --dev, --no-profile, and setup failure behavior

Fixes #122

Validation

• bash -n cli/bash/commands/basectl/subcommands/onboard.sh
• git diff --check
• ./bin/basectl onboard --dry-run --dev --no-profile
• bats cli/bash/commands/basectl/tests/basectl.bats
• bats cli/bash/commands/basectl/tests/setup.bats cli/bash/commands/basectl/tests/basectl.bats cli/bash/commands/caff/tests/caff.bats cli/bash/commands/sort-in-place/tests/sort-in-place.bats lib/bash/file/tests/lib_file.bats lib/bash/git/tests/lib_git.bats lib/bash/runtime/tests/runtime_bashrc.bats lib/bash/std/tests/lib_std.bats lib/bash/version/tests/lib_version.bats tests/install.bats
• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest