Fix pipeline deadlock on cross-team task dependencies

[Iron-Ham]

Summary

• Fixed pipeline deadlock: pipeline.Decompose grouped tasks into execution teams using only file-affinity (union-find on shared files). Tasks with DependsOn relationships but disjoint files landed in separate teams. Since TaskQueue.isClaimable() only resolves dependencies within its own task set, cross-team dependencies were permanently unsatisfiable — deadlocking the pipeline after the first dependency group completed.
• Added unknown dep ID guard: The dependency union loop now skips DependsOn entries referencing task IDs not in the plan, preventing silent union-find corruption via phantom root nodes.
• Renamed groupByFileAffinity → groupByAffinity to reflect its expanded responsibility (files + dependency edges).

Root Cause

The decomposer's union-find only unioned tasks sharing files. When task A (Swift files) and task B (GraphQL files) had no file overlap, they landed in different execution teams. Downstream tasks depending on both A and B were placed in teams where the dep IDs didn't exist in the local TaskQueue, causing isClaimable() to return false permanently.

Fix

Union tasks along DependsOn edges in addition to file edges during decomposition. This ensures all task-level dependencies are resolvable within a single team's queue.

Test plan

• TestDecompose_DependencyGrouping — direct dependency with disjoint files → same team
• TestDecompose_TransitiveDependencyGrouping — transitive chain (t3→t2→t1) with disjoint files → all in one team
• TestDecompose_DisjointDepsAndFiles — reproduces the exact real-world scenario (swipe framework + GraphQL codegen + wiring task)
• TestDecompose_UnknownDependencyIgnored — stale dep ID is skipped, no phantom union-find corruption
• All existing tests pass (no regressions)
• go test -race ./... passes
• go vet ./... clean
• gofmt -d . clean