Adjust tool usage notes (#6853)

Trying to work on some behaviors:

- Using `black` to format Python files (using `pre-commit` makes better
use of allow-listed commands)
- Writing Python code over 80 columns (adding more style notes)
- Running `bazel` (being more emphatic about `bazelisk`, splitting tool
usage out to its own skill to try making clear it's not
toolchain-specific)

Assisted-by: Google Antigravity with Gemini

.agents/skills/code_style/SKILL.md

@@ -21,18 +21,25 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 ## Formatting
 
--   **C++**: Always check `clang-format` on C++ files.
+-   **Bazel**: Use `pre-commit run buildifier --files <file.bzl>` to format
+    Bazel files.
+-   **C++**: Use `pre-commit run clang-format --files <file.cpp>` to format C++
+    files.
 -   **Carbon**: The toolchain's `format` command doesn't work well right now.
     Instead, try to format Carbon code based on other Carbon files and the C++
     style.
 -   **Markdown**: Use `pre-commit run prettier --files <file.md>` to format
-    markdown files correctly.
+    markdown files.
+-   **Python**: Use `pre-commit run black --files <file.py>` to format Python
+    files.
 
 ## Style Guides
 
-Carbon's toolchain uses LLVM-style C++ with some specific conventions.
-
--   **Style Guide**: Follow the
+-   **C++ style**: Follow the
     [Carbon C++ Project Style Guide](/docs/project/cpp_style_guide.md).
 -   **Markdown style**: Follow the
     [Google developer documentation style guide](https://developers.google.com/style).
+-   **Python style**: Follow the [PEP 8](https://peps.python.org/pep-0008/)
+    style guide.
+    -   Wrap code and comments to 80 columns.
+    -   Run `pre-commit run flake8 --files <file.py>` to check Python style.

.agents/skills/tool_usage/SKILL.md

@@ -0,0 +1,34 @@
+---
+name: Tool usage
+description:
+    Instructions for AI assistants on what tools to use in the carbon-lang
+    project.
+---
+
+# Tool usage
+
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+## Bazelisk and Bazel
+
+We use `bazelisk` for build and test.
+
+**IMPORTANT**: AI assistants use `bazelisk` instead of `bazel`.
+
+## Pre-commit
+
+Running `pre-commit` is mandatory. To run it on all files:
+
+```bash
+pre-commit run -a
+```
+
+To validate a specific list of files:
+
+```bash
+pre-commit run --files <files>
+```

.agents/skills/toolchain_development/SKILL.md

@@ -41,11 +41,6 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 -   **Iteration**: Prefer iterative algorithms over recursive ones to prevent
     stack exhaustion on complex codebases.
 
-## Building and testing
-
-AI assistants should use `bazelisk` instead of `bazel` for build and test
-commands, because some AI editors won't see `bazel` aliases.
-
 ### Essential commands
 
 -   **Test everything**: `bazelisk test //...`
@@ -67,20 +62,6 @@ of expected output.** Use the script:
 ./toolchain/autoupdate_testdata.py toolchain/check/testdata/my_test.carbon
 ```
 
-### Pre-commit
-
-Running `pre-commit` is mandatory. To run it on all files:
-
-```bash
-pre-commit run -a
-```
-
-To validate a specific list of files:
-
-```bash
-pre-commit run --files <files>
-```
-
 ## Debugging and diagnostics
 
 -   **Printing to stderr**: Use `llvm::errs() << "debug info\n";`.

GEMINI.md

@@ -35,6 +35,11 @@ assistant, **read this first** to avoid common pitfalls.
 -   **[`toolchain/`](toolchain/)**: The C++ implementation of the compiler
     (Toolchain).
 
+## Tool usage
+
+See the "Tool usage" skill for instructions on what tools to use in the
+carbon-lang project.
+
 ## Code style
 
 See the "Code style" skill for instructions on formatting, style guides, and