Provides robust parsing and manipulation of Git commit message trailers (subject, body, and key-value trailers). Uses a finite-state machine approach rather than fragile regex, correctly handling continuation lines and multiple trailers. The module is self-contained (no ghstack-specific imports) and can be extracted as a reusable library for any tool that needs to parse or modify git commit message trailers.

Implements a simple, durable key-value cache backed by SQLite with time-based eviction. The cache is parameterized by a table name, making it suitable for caching different types of data (e.g., CircleCI build results). It handles concurrent access with a single global connection (not thread-safe but adequate for synchronous usage). The pattern is straightforward and could be extracted for any project needing a persistent disk cache with minimal dependencies.

Defines the abstract `GitHubEndpoint` class that declares the async interface for all GitHub API interactions (PR creation/update, querying, etc.). This pattern enables the testing infrastructure with `FakeGitHubEndpoint` without requiring network calls. The interface is clean, with each method returning typed dataclasses. While the method signatures are specific to ghstack's domain, the abstract-interface pattern itself is reusable for any project needing mockable API clients.