Decompose architecture into 28 atomic implementation tasks

Break the @alkdev/taskgraph architecture specs into dependency-ordered
implementation tasks across 8 component directories: setup, schema,
error, graph, analysis, cost-benefit, frontmatter, api, and review.
Each task has clear acceptance criteria referencing specific architecture
docs. Three review tasks serve as quality gates at critical junction
points (schemas-and-errors, graph-complete, complete-library). The
dependency graph is validated acyclic with 9 topological levels enabling
significant parallelism across independent work streams.

tasks/implementation/analysis/critical-path.md

@@ -0,0 +1,40 @@
+---
+id: analysis/critical-path
+name: Implement criticalPath and weightedCriticalPath functions
+status: pending
+depends_on:
+  - graph/construction
+  - graph/queries
+scope: moderate
+risk: medium
+impact: component
+level: implementation
+---
+
+## Description
+
+Implement `criticalPath` and `weightedCriticalPath` as standalone functions. `criticalPath` finds the longest path from sources to sinks using default edge weighting. `weightedCriticalPath` accepts a custom weight function for per-node weighting.
+
+`criticalPath` can be implemented via topological order + dynamic programming (longest path in DAG). For unweighted, each edge has weight 1; for weighted, each node contributes a weight.
+
+## Acceptance Criteria
+
+- [ ] `criticalPath(graph: TaskGraph): string[]` — returns the longest path as an ordered array of task IDs
+- [ ] `weightedCriticalPath(graph: TaskGraph, weightFn: (taskId: string, attrs: TaskGraphNodeAttributes) => number): string[]` — returns the path with the highest cumulative weight
+- [ ] Both functions throw `CircularDependencyError` if graph is cyclic
+- [ ] When multiple paths tie, returns any one of them (deterministic order preferred)
+- [ ] Empty graph returns `[]`; single-node graph returns `[nodeId]`
+- [ ] Unit tests: linear chain (the chain itself is critical path), diamond graph (tests path selection), weighted variant with diverse scope values
+
+## References
+
+- docs/architecture/api-surface.md — criticalPath, weightedCriticalPath signatures
+- docs/architecture/graph-model.md — edge direction (prerequisite→dependent determines source→sink)
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion