PR graph and dependencies

The implementation plan is a directed graph. Nodes are PRs. Edges are dependencies.

Rules for the graph:

• Each PR has one deployment target label.
• Each PR should be small enough to review as one implementation slice.
• A PR is Ready when all its dependencies are merged.
• Multiple Ready PRs can be implemented in parallel by different agents or different engineers.

PR statuses, in order:

1. Blocked
2. Ready
3. Implementing
4. In review
5. Merged