ci: reproducible deepagent tests + venv workflow (#23)

* ci: add reproducible test runner, fix requirements pins, use venv in workflow

* chore(ci): add Makefile, README test docs, run ruff in CI; remove .venv_ci

* docs: fix README markdown warnings; ci: add codecov token and pip cache for provider jobs

* ci: exclude duplicate router from mypy checks (known conflict)

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Co-authored-by: SDLC Bot <devnull@example.com>

Author: vinod0m