feat(analysis/parallel-groups): implement parallelGroups function with tests

tasks/implementation/analysis/parallel-groups.md

@@ -1,7 +1,7 @@
 ---
 id: analysis/parallel-groups
 name: Implement parallelGroups analysis function
-status: pending
+status: completed
 depends_on:
   - graph/construction
   - graph/queries
@@ -17,12 +17,12 @@ Implement `parallelGroups(graph: TaskGraph): string[][]` in `src/analysis/index.
 
 ## Acceptance Criteria
 
-- [ ] `parallelGroups` returns `string[][]` where each inner array is a generation of tasks at the same depth from sources
-- [ ] Uses `graphology-dag.topologicalGenerations()` for the generation computation
-- [ ] Tasks with zero prerequisites are in the first group
-- [ ] Throws `CircularDependencyError` if the graph is cyclic (delegated to `topologicalGenerations` behavior)
-- [ ] Works on disconnected graphs (each connected component sorted independently, then merged by depth)
-- [ ] Unit tests: linear chain (each group size 1), diamond graph, disconnected components
+- [x] `parallelGroups` returns `string[][]` where each inner array is a generation of tasks at the same depth from sources
+- [x] Uses `graphology-dag.topologicalGenerations()` for the generation computation
+- [x] Tasks with zero prerequisites are in the first group
+- [x] Throws `CircularDependencyError` if the graph is cyclic (delegated to `topologicalGenerations` behavior)
+- [x] Works on disconnected graphs (each connected component sorted independently, then merged by depth)
+- [x] Unit tests: linear chain (each group size 1), diamond graph, disconnected components
 
 ## References
 
@@ -31,8 +31,11 @@ Implement `parallelGroups(graph: TaskGraph): string[][]` in `src/analysis/index.
 
 ## Notes
 
-> To be filled by implementation agent
+Implementation uses `topologicalGenerations` from `graphology-dag` which internally uses Kahn's algorithm. It naturally handles disconnected graphs by grouping source nodes (zero in-degree) from all components into the same first generation. On cyclic graphs, `topologicalGenerations` throws and we catch it to re-throw `CircularDependencyError` with cycle information (same pattern as `topologicalOrder` in queries.ts).
 
 ## Summary
 
-> To be filled on completion
+Implemented `parallelGroups(graph: TaskGraph): string[][]` in a dedicated module.
+- Created: `src/analysis/parallel-groups.ts`, `test/parallel-groups.test.ts`
+- Modified: `src/analysis/index.ts` (added re-export)
+- Tests: 14, all passing (full suite: 457 tests passing, lint clean)