Motivation

• The previous build_plan implementation was large and mixed multiple responsibilities (metadata loading, publishability checks, graph construction, selection, sorting, and plan assembly), making it hard to reason about and test.
• Split responsibilities into single-responsibility modules to improve clarity, testability, and future maintainability.

Description

• Replace the long build_plan body with a short orchestration that delegates to focused submodules: metadata, publishability, graph, selection, and assembly, and wire their APIs together via build_plan.
• Add crates/shipper-core/src/plan/metadata.rs to isolate the cargo metadata boundary and load_metadata call used by planning.
• Add crates/shipper-core/src/plan/publishability.rs to compute the publishable workspace set, record skipped packages, and encapsulate publish_allowed/skip-reason logic.
• Add crates/shipper-core/src/plan/graph.rs to construct manifest-level workspace dependency edges, validate non-publishable deps, and provide the deterministic topo_sort implementation.
• Add crates/shipper-core/src/plan/selection.rs to resolve selected-package inclusion and transitively include internal dependencies.
• Add crates/shipper-core/src/plan/assembly.rs to build PlannedPackage values, produce the dependency projection map, and compute the stable plan_id hash.
• Update crates/shipper-core/src/plan/mod.rs to use the new modules and to assemble the final PlannedWorkspace from their outputs while preserving existing behavior and test surface.

Testing

• Ran cargo check -p shipper-core and it completed successfully.
• Ran cargo test -p shipper-core plan and the plan-related test suite completed successfully (all tests passed).
• Ran formatting and style checks with cargo fmt --all -- --check and the code is properly formatted (check passed).

Codex Task