docs: add current implementation agent guide

[vietanhdev]

Summary

• add a canonical AGENT.md covering the current UI, auto-labeling, accelerator, packaging, testing, and release architecture
• make CLAUDE.md load the shared guide and retain Claude-specific validation requirements
• update the README stable release section to v0.4.43
• remove the obsolete setup.py packaging guidance and document the current PEP 621 GPU rewrite

Validation

• QT_QPA_PLATFORM=offscreen .venv/bin/python -m unittest discover -s tests -v (142 passed, 13 optional model skips)
• ruff check anylabeling --exclude anylabeling/resources/resources.py
• ruff format --check anylabeling --exclude anylabeling/resources/resources.py
• git diff --check
• verified every path and workflow named in AGENT.md exists

The unrelated local .gitignore change was intentionally excluded.