Improve feedback used by agentic mode

[lucaminudel]

Based on what one is doing, that feedback may arrive by other means, such as the build or the tests (an html page snapshot created before the failure of an e2e test or a page screenshot) and even the build and the tests may be better executed with a different command that Gemini CLI would use by default.

It would be useful to let the user describe the commands that generate relevant feedback for the agentic mode or their file system source, and the ability to select them based on the prompt the user is going to send.

For example, if I am fixing a visualisation bug in a web page, where there is an e2e test, I may direct Gemini to the command to execute the failing e2e test and the path to look for the html that is the snapshot of the page that did not pass the test