Instruct agents to create a repo root alias (#10165)

In my trials so far, Jules struggles to run the repo tooling commands once it has moved into a target directory, because it frequently tries to use a root-relative path instead of an absolute path. To try to reduce that problem, this instructs the agent to explicitly create an alias so that the later commands that reference REPO_ROOT as a placeholder will, hopefully, be run exactly as written in the examples instead.

Also, to reduce unnecessary working directory confusion, switch to `dart pub get -C` instead of `cd` followed by `dart pub get` for initializing the tooling. (Also makes the same change to the tool README, because why not have humans run one command instead of three.)

Author: stuartmorgan-g

AGENTS.md

@@ -40,10 +40,15 @@ The primary tool for this repository is `flutter_plugin_tools.dart`.
 
 ### Initial Setup
 
-First, initialize the tooling:
+First, define an environment variable for the repository root directory and initialize the tooling:
 ```bash
-cd $REPO_ROOT/script/tool # $REPO_ROOT is the repository root
-dart pub get
+# Define an environment variable for the repository root.
+export REPO_ROOT=$(pwd)
+
+# Verify that the environment variable is working correctly.
+echo "Repository root directory: $REPO_ROOT"
+
+dart pub get -C $REPO_ROOT/script/tool
 ```
 
 ### Identifying Target Packages

script/tool/README.md

@@ -8,7 +8,7 @@ local development.
 Set up:
 
 ```sh
-cd script/tool && dart pub get && cd ../../
+dart pub get -C script/tool
 ```
 
 Run: