Add CI workflow for documented checks

[GHX5T-SOL]

Summary

• Add a GitHub Actions CI workflow for the documented lint, typecheck, and test loop on main pushes and pull requests.
• Run the workflow on Python 3.11 and 3.12 with pip caching keyed from pyproject.toml and optional requirements*.txt files.
• Add a README badge for the new workflow.
• Add the missing type annotations needed for make typecheck to pass under the current strict mypy setup.

Closes #3

Validation

• PATH="$PWD/.venv/bin:$PATH" make lint - passed
• PATH="$PWD/.venv/bin:$PATH" make typecheck - Success: no issues found in 19 source files
• PATH="$PWD/.venv/bin:$PATH" make test - 29 passed
• .venv/bin/python YAML parse of .github/workflows/ci.yml - passed
• PATH="$PWD/.venv/bin:$PATH" black --check verity/api/main.py verity/config.py verity/observability/logging.py - passed
• git diff --check HEAD~1 HEAD - passed
• gitleaks detect --source . --log-opts HEAD~1..HEAD --redact --no-banner - no leaks found

Notes

• Before the type-annotation cleanup, make typecheck failed locally with three strict mypy errors. The workflow would have reported the same failure, so this PR includes those minimal annotation fixes with the CI setup.