Add lazy entry point extension discovery

[codeforester]

Summary

• add lazy, deterministic ExtensionDiscovery for base_cli.commands, base_cli.profiles, and base_cli.plugins
• expose distribution/version/extras metadata with explicit duplicate-name errors and per-instance caching
• support allowlists, complete disablement, refresh, and isolated bulk-load failures
• document the entry-point contracts and add real temporary-distribution, collision, cache, allowlist, disablement, and failure tests

Validation

• .venv/bin/python -m pytest -q
• .venv/bin/python -m mypy lib/python/base_cli/extensions.py --ignore-missing-imports
• git diff --check

Closes #64