docs: Add note about clang-tidy installation (#6634)

CONTRIBUTING.md

@@ -259,6 +259,10 @@ There is a Continuous Integration job that runs clang-tidy on pull requests. The
 
 This ensures that configuration changes don't introduce new warnings across the codebase.
 
+### Installing clang-tidy
+
+See the [environment setup guide](./docs/build/environment.md#clang-tidy) for platform-specific installation instructions.
+
 ### Running clang-tidy locally
 
 Before running clang-tidy, you must build the project to generate required files (particularly protobuf headers). Refer to [`BUILD.md`](./BUILD.md) for build instructions.
@@ -266,10 +270,15 @@ Before running clang-tidy, you must build the project to generate required files
 Then run clang-tidy on your local changes:
 
 ```
-run-clang-tidy -p build src tests
+run-clang-tidy -p build src include tests
 ```
 
-This will check all source files in the `src` and `tests` directories using the compile commands from your `build` directory.
+This will check all source files in the `src`, `include` and `tests` directories using the compile commands from your `build` directory.
+If you wish to automatically fix whatever clang-tidy finds _and_ is capable of fixing, add `-fix` to the above command:
+
+```
+run-clang-tidy -p build -fix src include tests
+```
 
 ## Contracts and instrumentation
 

docs/build/environment.md

@@ -109,3 +109,32 @@ Install CMake with Homebrew too:
 ```
 brew install cmake
 ```
+
+## Clang-tidy
+
+Clang-tidy is required to run static analysis checks locally (see [CONTRIBUTING.md](../../CONTRIBUTING.md)).
+It is not required to build the project. Currently this project uses clang-tidy version 21.
+
+### Linux
+
+LLVM 21 is not available in the default Debian 12 (Bookworm) repositories.
+Install it using the official LLVM apt installer:
+
+```
+wget https://apt.llvm.org/llvm.sh
+chmod +x llvm.sh
+sudo ./llvm.sh 21
+sudo apt install --yes clang-tidy-21
+```
+
+Then use `run-clang-tidy-21` when running clang-tidy locally.
+
+### macOS
+
+Install LLVM 21 via Homebrew:
+
+```
+brew install llvm@21
+```
+
+Then use `run-clang-tidy` from the LLVM 21 Homebrew prefix when running clang-tidy locally.