[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 in runtime modules.
• Cover shard parsing, bounded cache behavior, Markdown parsing, regex behavior, Twitter proxy selection, grouping, locale files, migrations, and Compose topology.

Priority

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

Dependency

Stacked on #3. Review and merge #3 first; after it merges, retarget this PR to main.

Validation

• Ruff passes.
• Pytest passes: 16 tests.
• All Python files pass AST parsing.

[apple050620312]

Superseded by upstream PR Kyrela#114.