[P3] Add CI coverage and reproducible dependencies

[apple050620312]

Summary

• Pin Git-based dependencies to immutable commit revisions.
• Add separate development dependencies and centralized pytest/Ruff configuration.
• Add GitHub Actions checks for linting and the test suite.
• Replace wildcard imports with explicit imports and make required standard-library imports explicit.
• Cover Markdown parsing, route regex behavior, Twitter proxy selection, grouping, and locale files.

Priority

P3 — improves long-term maintainability, regression detection, and reproducible builds after production risks are addressed.

Validation

• Ruff passes.
• Pytest passes: 5 tests.
• All Python files pass AST parsing.