feat: formalize extension SDK compatibility

[codeforester]

Summary

• add typed command, profile, and plugin callback protocols
• add extension API version and capability metadata from standard entry-point extras
• reject unsupported API versions before loading an extension
• document the plugin SDK and add negotiation/collision regression coverage

Validation

• python -m pytest (all tests passed)
• ruff check and ruff format --check
• python -m mypy --strict lib/python/base_cli
• scripts/validate_docs.py
• ./tests/validate.sh
• git diff --check

Closes #193