Add initial flake8 HTK rules plugin

[jontsai]

Summary

• extract the accounts-django structured-programming checker shape into the standalone plugin
• add SP100 and SP101, gated by --structured-programming-files
• broaden SP101 so literal displays match the public docs
• add datetime clarity rules DT100-DT102
• add debugger prevention rules DB100 and DB101 for debugger imports and calls
• add naming rule NM100 to warn on vague get_ function/method prefixes
• split check implementation into thematic modules under src/flake8_htk_rules/checks/
• add real Flake8 entry-point integration coverage and CI flake8 linting
• update README/package metadata and CI branch targeting for master
• remove internal decision-log material from the public OSS package

Verification

• temp virtualenv: python -m unittest discover -s tests -v -> 20 passed
• temp virtualenv: python -m pytest -> 20 passed
• temp virtualenv: python -m flake8 src tests
• temp virtualenv: python -m build --wheel and wheel contents include flake8_htk_rules/checks/ modules
• integration test runs python -m flake8 and verifies DT100, SP100, SP101, DB100, DB101, and NM100 via the installed entry point