Add guideline for defining the name of a controller

[mcmire]

Explanation

There is no written standard for defining the name of a controller, and some contributors do not have a common understanding of the do's and dont's for the name.

This commit adds such a guideline, highlighting the following points:

• The name of a controller should be defined in a variable called CONTROLLER_NAME (formerly controllerName)
• The name should be used to define custom actions and events and should be passed to BaseController
• The name should not be exported from the package

Besides updating the guidelines, this commit also updates the sample-controllers package to follow them, most notably changing controllerName to CONTROLLER_NAME and removing the export from this constant.

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Low Risk
Low risk documentation and sample-code refactor; the only potential impact is for any downstream code importing controllerName directly from sample controller source files rather than via the package’s public exports.

Overview
Adds a new controller guideline recommending an internal CONTROLLER_NAME constant to namespace messenger actions/events and controller state, and explicitly discouraging exporting that constant from the package.

Updates the sample-controllers examples to follow the guideline by renaming controllerName to CONTROLLER_NAME, making it non-exported, and wiring all related Messenger/BaseController type parameters and name initialization to the new constant.

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[mcmire]

Adding no-changelog because this is a non-public-facing change to sample-controllers.