UI text: Update flow text

Problem to solve

We should update the text of these foundational flows to better match GitLab UI text standards.

The full descriptions are displayed when you select Automate > Agents. We should write these descriptions so the snippet that's displayed in the Chat window is helpful.

(Note that we always use GitLab with Duo. So either we drop GitLab Duo altogether, or we add it.)

Proposal

General development agent

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Epic/Issue summarizer

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Planner

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