DOC: Address PR review comments in building.md and README.md

- Add pixi install commands to building.md with explanatory comments
- Replace pixi.toml reference with pyproject.toml
- Remove placeholder rows from Documentation/AI/README.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hjmjohnson

Documentation/AI/README.md

@@ -25,8 +25,6 @@ Add file to table in AGENTS.md for routing:
 |------|-------------|
 | `building.md` | Configuring or building ITK with Pixi or CMake |
 | `testing.md` | Writing, running, or converting tests |
-| `new_task.md` | Conditions for including context from `new_task.md` |
-|   ...         | .... |
 
 ## Adding a New Context File
 

Documentation/AI/building.md

@@ -2,10 +2,17 @@
 
 ## Recommended: Pixi
 
-Pixi is the preferred build tool for development. It manages the toolchain and
-runs CMake + Ninja internally.
-
 ```bash
+# Pixi is the preferred build tool for development.
+# It manages the toolchain and runs CMake + Ninja internally.
+#
+# IMPORTANT:
+# Run each install command once after cloning the repository (or after lockfile changes).
+# Required one time before any `pixi run --as-is` commands.
+pixi install                    # Install the default locked toolchain environment
+pixi install -e cxx             # Install the cxx environment
+pixi install -e python          # Install the python environment
+
 pixi run --as-is build          # Build C++ tests
 pixi run --as-is build-python   # Build Python tests
 pixi run --as-is test           # Run C++ tests
@@ -15,7 +22,7 @@ pixi shell -e cxx               # Interactive C++ dev shell
 pixi shell -e python            # Interactive Python dev shell
 ```
 
-See `pixi.toml` for the full list of tasks and environment definitions.
+See `pyproject.toml` for the full list of tasks and environment definitions.
 
 ## Direct CMake