Add TsqlAstParser documentation to CLAUDE.md

Document how to use the TsqlAstParser C# tool to generate ast.json
files for test cases. Include prerequisites (.NET SDK, NuGet package),
usage examples, and limitations (unsupported SQL Server 170 features,
Fabric DW syntax, deprecated/error test cases).

Author: claude

CLAUDE.md

@@ -64,3 +64,52 @@ Each test in `parser/testdata/` contains:
 ## Important Rules
 
 - **NEVER modify `ast.json` files** - These are golden files containing the expected output. If tests fail due to JSON mismatches, fix the Go code to match the expected output, not the other way around.
+
+## Generating ast.json with TsqlAstParser
+
+The `TsqlAstParser/` directory contains a C# tool that generates `ast.json` files using Microsoft's official T-SQL parser (ScriptDom).
+
+### Prerequisites
+
+1. Install .NET 8.0 SDK:
+   ```bash
+   curl -sSL https://dot.net/v1/dotnet-install.sh | bash /dev/stdin --channel 8.0 --install-dir ~/.dotnet
+   ```
+
+2. Download the NuGet package (if `packages/` directory is empty):
+   ```bash
+   mkdir -p packages
+   curl -L -o packages/microsoft.sqlserver.transactsql.scriptdom.170.128.0.nupkg \
+     "https://api.nuget.org/v3-flatcontainer/microsoft.sqlserver.transactsql.scriptdom/170.128.0/microsoft.sqlserver.transactsql.scriptdom.170.128.0.nupkg"
+   ```
+
+3. Build the tool:
+   ```bash
+   ~/.dotnet/dotnet build TsqlAstParser -c Release
+   ```
+
+### Usage
+
+Generate `ast.json` for a single test:
+```bash
+~/.dotnet/dotnet run --project TsqlAstParser -c Release -- parser/testdata/TestName/query.sql parser/testdata/TestName/ast.json
+```
+
+Generate `ast.json` for all tests missing it:
+```bash
+for dir in parser/testdata/*/; do
+  if [ -f "$dir/query.sql" ] && [ ! -f "$dir/ast.json" ]; then
+    ~/.dotnet/dotnet run --project TsqlAstParser -c Release -- "$dir/query.sql" "$dir/ast.json"
+  fi
+done
+```
+
+### Limitations
+
+TsqlAstParser uses TSql160Parser (SQL Server 2022) and cannot parse:
+- SQL Server 170+ features (VECTOR indexes, AI functions, JSON enhancements)
+- Fabric DW-specific syntax (CLONE TABLE, CLUSTER BY)
+- Deprecated syntax removed in newer versions
+- Intentionally invalid SQL (error test cases)
+
+Tests for unsupported syntax will not have `ast.json` files generated.