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/build-distribution.md

@@ -51,7 +51,7 @@ taskgraph_ts/
 │   │   ├── parse.ts           # YAML/frontmatter parsing + typebox validation
 │   │   └── serialize.ts       # TaskInput → markdown with frontmatter
 │   └── error/
-│       └── index.ts           # TaskgraphError, TaskNotFoundError, CircularDependencyError, InvalidInputError
+│       └── index.ts           # TaskgraphError, TaskNotFoundError, CircularDependencyError, InvalidInputError, DuplicateNodeError, DuplicateEdgeError
 ├── test/
 │   ├── graph.test.ts
 │   ├── analysis.test.ts