Fix critical architecture review issues

Critical fixes:
- Rename qualityDegradation → qualityRetention across all docs
  (semantically inverted: 0.9 meant 90% quality RETAINED, not 90%
  degradation). Updated schemas, graph-model, cost-benefit, ADRs.
- Add TaskInput → TaskGraphNodeAttributes transformation section
  to graph-model.md, documenting how Nullable(Optional) input fields
  map to Optional graph attributes
- Fix DuplicateEdgeError fields: source/target → prerequisite/dependent
  to match the established edge direction convention
- Fix resolveDefaults signature: Partial<TaskGraphNodeAttributes>
  → Partial<...> & Pick<TaskGraphNodeAttributes, 'name'> to
  require the name field
- Move Nullable helper definition before its first use in schemas.md
- Fix 'construction never throws' contradiction: rephrase to
  'construction enforces uniqueness, not data quality'
- Define all 6 enum value sets in schemas.md (previously only
  TaskScope and TaskRisk were explicit)
- Add EvConfig parameter table with defaults and semantics
- Document WorkflowCostOptions.limit parameter
- Add construction error handling table to graph-model.md
- Add graph.raw mutation safety warning to api-surface.md
- Update build-distribution.md error class list to include
  DuplicateNodeError and DuplicateEdgeError

docs/architecture/api-surface.md

@@ -50,9 +50,10 @@ class TaskGraph {
 
   // Reactivity
   get raw(): Graph  // underlying graphology instance for direct event listener attachment
-}
 ```
 
+**Warning on `graph.raw`**: Mutating the underlying graphology instance directly bypasses `TaskGraph`'s validation and invariants. Operations that violate the "no parallel edges" constraint (adding a second edge between the same node pair without using `addEdgeWithKey`), or that create self-loops in a graph configured to disallow them, will not be caught by `TaskGraph` methods. Consumers using `raw` should treat the graph as read-only for structural changes and use `TaskGraph` methods for all mutations.
+
 **Notes**:
 - `topologicalOrder()` throws `CircularDependencyError` (with `cycles` populated) when cyclic — see [ADR-003](decisions/003-topo-order-throws-on-cycle.md)
 - `subgraph()` returns a new `TaskGraph` with matching nodes and only edges where both endpoints are in the filtered set — see [ADR-007](decisions/007-subgraph-internal-only.md)
@@ -131,12 +132,12 @@ const DecomposeResult = Type.Object({
 
 ```typescript
 const WorkflowCostOptions = Type.Object({
-  includeCompleted: Type.Optional(Type.Boolean()),
-  limit: Type.Optional(Type.Number()),
+  includeCompleted: Type.Optional(Type.Boolean()),  // default: false. When false, completed tasks are excluded from output but remain in propagation with p=1.0
+  limit: Type.Optional(Type.Number()),               // default: no limit. If set, limits the number of tasks in the result. Useful for large graphs when only top-N results are needed.
   propagationMode: Type.Optional(
     Type.Union([Type.Literal("independent"), Type.Literal("dag-propagate")])
   ),
-  defaultQualityDegradation: Type.Optional(Type.Number()),
+  defaultQualityRetention: Type.Optional(Type.Number()),  // default: 0.9. Edge quality when not explicitly set.
 })
 ```