Add README for Quantum Metric Schema

Author: zachrenwick

README.md

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+# Quantum Metric Schema Tree
+
+Interactive collapsible tree visualization of the BigQuery schema for `quantummetrics_prd.sessions_archive`.
+
+## Usage
+
+Open `quantum_metric_schema.html` directly in a browser (`file://` or served via HTTP). No build step required.
+
+- **Click** a node to expand/collapse
+- **Scroll** to zoom
+- **Drag** to pan
+- **Right-click** to reset view
+
+## Details
+
+- 297 fields, max nesting depth 4
+- Built with [ECharts](https://echarts.apache.org/) (CDN)
+- Self-contained — schema data is embedded in the HTML
+
+## Refreshing the Schema
+
+1. Query BigQuery table metadata using the MCP BigQuery tool (`get_table_info`) or `bq show`.
+2. Transform fields into `{ name: "field_name (TYPE)", children: [...] }` format.
+3. Replace the `SCHEMA_DATA` constant in the HTML file.