Add CI improvements flow for DAP onboarding

What does this MR do and why?

Implements the GitLab CI Improvements Flow for the Duo Agent Platform onboarding page (issue #599014 (closed)), following the same controller/frontend pattern established in !229847 (merged) for the Project Context Init Flow.

When a user visits the Automate > Onboarding page, a new "Improve CI setup" section is shown. If the project has a .gitlab-ci.yml on the default branch, a button is displayed to launch an "Improve CI" task

References

• Issue: #599014 (closed)
• Context Init Flow MR (base): !229847 (merged)

Screenshots or screen recordings

Before	After
Onboarding page with only "Initialize project context"	Onboarding page with both "Initialize project context" and "Improve CI setup" sections

How to set up and validate locally

1. Enable the feature flag in the Rails console:

   Feature.enable(:duo_agent_onboarding)

2. Navigate to any project's Automate > Onboarding page: http://127.0.0.1:3000/<namespace>/<project>/-/automate/onboarding
3. Verify the "Improve CI setup" section appears.
4. If the project has a .gitlab-ci.yml, verify the "Improve CI setup" button is shown.
5. If the project has no .gitlab-ci.yml, verify a warning alert is shown instead of the button.
6. Click the button and verify it calls the improve_ci endpoint and shows a link to the agent session.

MR acceptance checklist

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