feat: add aggregation engine with incremental SUM/COUNT/AVG/MIN/MAX, GROUP BY, and time windows

Add a write-time incremental aggregation engine that maintains running
totals on every add/update/delete for O(1) read performance. Integrates
into brain.find({ aggregate }) for a unified query API.

Core features:
- AggregationIndex with defineAggregate()/removeAggregate() API
- Five aggregation operations: SUM, COUNT, AVG, MIN, MAX
- GROUP BY with multiple dimensions including time windows
- Time window bucketing: hour, day, week, month, quarter, year, custom
- Materialization of results as NounType.Measurement entities
- Debounced persistence of definitions and state to storage
- Definition change detection via FNV-1a hashing with auto-rebuild
- Infinite loop prevention for materialized entities
- 'aggregation' plugin provider key for native acceleration
- Lazy initialization (created on first defineAggregate() call)
- 73 tests (unit + integration) covering all functionality

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude/skills/architecture.md

@@ -78,6 +78,14 @@
 - BrainyMCPAdapter, MCPAugmentationToolset, BrainyMCPService
 - Model Control Protocol request/response handling
 
+### Aggregation Engine (`src/aggregation/`)
+- **AggregationIndex** (`AggregationIndex.ts`): Write-time incremental aggregation — SUM, COUNT, AVG, MIN, MAX with GROUP BY and time windows
+- **Time Windows** (`timeWindows.ts`): ISO 8601 bucketing — hour, day, week, month, quarter, year, custom intervals
+- **Materializer** (`materializer.ts`): Debounced writes of aggregate results as `NounType.Measurement` entities
+- Integrates into `brain.find({ aggregate })` for unified query API
+- Write hooks in `add()`, `update()`, `delete()` for O(1) incremental updates
+- `'aggregation'` provider key enables native plugin acceleration
+
 ### Additional Systems
 - **CLI** (`src/cli/`): Complete command-line tool with interactive mode and catalog system
 - **Migration** (`src/migration/`): MigrationRunner for database schema migrations