Instruct agents to create a repo root alias

[stuartmorgan-g]

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.)

[gemini-code-assist]

Code Review

This pull request updates setup instructions in AGENTS.md and script/tool/README.md. It introduces a command to define a REPO_ROOT environment variable in AGENTS.md. It also replaces a multi-command sequence involving cd and dart pub get with a single dart pub get -C command in both files. I have provided one comment to suggest a terminology clarification in the documentation for accuracy.

[gemini-code-assist]

For improved accuracy, it's better to call REPO_ROOT an 'environment variable' instead of an 'alias'. In shell scripting, an alias is a shortcut for a command, whereas export creates an environment variable that holds a value. This change updates the terminology in both the description and the code comment for correctness.

Suggested change

	First, define an alias 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 alias for the repository root.
	First, define an environment variable for the repository root directory and initialize the tooling:
	```bash
	# Define an environment variable for the repository root.

[stuartmorgan-g]

Entertainingly, the use of "alias" came from Gemini in the first place 🤷🏻 I left it alone before because it was a Gemini suggestion, so I was trusting the model, but it seems like maybe it was a quirk of my prompt rather than a model preference so I've changed it to be accurate.