Create include-cleaner skill

Bug: webrtc:465491622
Change-Id: I277172143a77a6ea88102ae1a9668f476a6a6964
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/460020
Commit-Queue: Evan Shrubsole <eshr@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47277}

.agents/skills/include-cleaner/SKILL.md

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+---
+name: webrtc-include-cleaner
+description: Runs the WebRTC include-cleaner tool (IWYU replacement) to fix headers in C++ files. Use when preparing a CL for upload, after modifying .cc or .h files, or when instructed to fix include regressions.
+---
+
+# WebRTC Include Cleaner
+
+This skill provides instructions for using
+`tools_webrtc/iwyu/apply-include-cleaner`, a tool that automatically manages C++
+`#include` directives in the WebRTC codebase. It ensures that every header used
+is explicitly included and that unused headers are removed.
+
+## When to Use
+
+- **Pre-upload**: Run this tool before uploading a CL to ensure clean includes.
+- **After refactoring**: When moving code or changing dependencies, use this to
+  update `#include` blocks.
+- **Fixing regressions**: Use this if a presubmit or bot identifies
+  include-related issues.
+
+## Basic Usage
+
+To run the include cleaner on specific files:
+
+```bash
+tools_webrtc/iwyu/apply-include-cleaner path/to/file.cc path/to/file.h
+```
+
+To run it on all modified files relative to the upstream branch (ideal for CL
+preparation):
+
+```bash
+tools_webrtc/iwyu/apply-include-cleaner
+```
+
+Note: This is as expensive as a build for each file, so use it sparingly.
+
+## Options
+
+- `-p`, `--print`: Don't modify the files, just print the proposed changes.
+- `-w WORK_DIR`, `--work-dir WORK_DIR`: Specify the GN work directory (default:
+  `out/Default`).
+
+## Post-Execution Steps
+
+After running the include cleaner, it is recommended to perform the following
+steps to ensure build and style consistency:
+
+1. **Check for build errors**: The tool might occasionally make mistakes. Run a
+   build to verify.
+1. **Fix GN dependencies**: Use `tools_webrtc/gn_check_autofix.py` to fix any
+   `deps` issues caused by include changes.
+   ```bash
+   tools_webrtc/gn_check_autofix.py -C out/Default
+   ```
+1. **Format code**: Run `git cl format` to fix any formatting issues in the
+   `#include` blocks.
+   ```bash
+   git cl format
+   ```
+
+## Prerequisites
+
+- The tool automatically generates `compile_commands.json` in the output
+  directory if `out/Default` exists.
+- `clangd` must be checked out in your `.gclient` file
+  (`"checkout_clangd": True`).