feat: Expand data category with 4 advanced dataflow skills

Added comprehensive skills for timely-dataflow and differential-dataflow systems:

- timely-dataflow.md: Timely dataflow framework, progress tracking, operators
  * Core concepts: dataflow graphs, timestamps, frontiers
  * Patterns: custom operators, stateful aggregation, iterative computation
  * Rust examples with timely crate

- differential-dataflow.md: Differential computation, incremental updates
  * Core concepts: collections, arrangements, differential operators
  * Patterns: incremental joins, group-by reduce, connected components
  * Efficient arrangements for shared state

- dataflow-coordination.md: Coordination patterns for distributed systems
  * Core concepts: barriers, epochs, watermarks, snapshots
  * Patterns: Chandy-Lamport snapshots, backpressure, causal consistency
  * Multi-language examples (Rust, Go, Python)

- streaming-aggregations.md: Windowing and time-series aggregation
  * Core concepts: tumbling/sliding/session windows, watermarks
  * Patterns: late data handling, multi-resolution aggregation, top-K
  * Time semantics and watermark strategies

Updated INDEX.md from 5 to 9 skills and enhanced discover-data gateway
with workflow combinations for real-time analytics and incremental
computation systems.

Author: rand

skills/data/INDEX.md

@@ -2,7 +2,7 @@
 
 ## Category Overview
 
-**Total Skills**: 5
+**Total Skills**: 9
 **Category**: data
 
 ## Skills in This Category
@@ -27,6 +27,26 @@ cat skills/data/data-validation.md
 
 ---
 
+### dataflow-coordination.md
+**Description**: Coordination patterns for distributed dataflow systems including barriers, epochs, and distributed snapshots
+
+**Load this skill**:
+```bash
+cat skills/data/dataflow-coordination.md
+```
+
+---
+
+### differential-dataflow.md
+**Description**: Differential computation for incremental updates, maintaining indexed collections and efficient joins
+
+**Load this skill**:
+```bash
+cat skills/data/differential-dataflow.md
+```
+
+---
+
 ### etl-patterns.md
 **Description**: Designing data extraction from multiple sources (databases, APIs, files)
 
@@ -57,6 +77,26 @@ cat skills/data/stream-processing.md
 
 ---
 
+### streaming-aggregations.md
+**Description**: Windowing, sessionization, time-series aggregation, and late data handling for streaming systems
+
+**Load this skill**:
+```bash
+cat skills/data/streaming-aggregations.md
+```
+
+---
+
+### timely-dataflow.md
+**Description**: Timely dataflow framework for low-latency, high-throughput streaming computation with progress tracking
+
+**Load this skill**:
+```bash
+cat skills/data/timely-dataflow.md
+```
+
+---
+
 ## Loading All Skills
 
 ```bash
@@ -67,7 +107,7 @@ ls skills/data/*.md
 cat skills/data/batch-processing.md
 cat skills/data/data-validation.md
 cat skills/data/etl-patterns.md
-# ... and 2 more
+# ... and 6 more
 ```
 
 ## Related Categories