Move build_agent.yaml PR creation from ddev CLI to a workflow_dispatch GitHub Action

[Kyle-Neale]

What does this PR do?

Moves ownership of the .gitlab/build_agent.yaml update from the ddev CLI to a GitHub Actions workflow.

Previously, ddev release branch tag would detect when build_agent.yaml still pointed to main, attempt to update it in-place, commit, and push directly to the protected release branch.

This PR replaces that approach with:

1. A new update-build-agent-yaml.yml workflow (workflow_dispatch) that checks out the release branch, verifies the agent branch exists in datadog-agent, updates build_agent.yaml, and opens a bot PR targeting the release branch— consistent with how other automated PRs in this repo are created.
2. A simplified pre-flight check in tag.py that reads build_agent.yaml and aborts with an actionable message if it still points to main, including the exact gh workflow run command to trigger the update.

Motivation

The CLI approach tried to push directly to a protected branch. The GitHub Actions approach uses a bot token (via actions/create-github-app-token) to open a PR, which is the established pattern in this repo (update-dependencies.yml,
update-agent-changelog.yml, etc.).

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

❌ Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.53%. Comparing base (391b541) to head (3717e0d).
⚠️ Report is 19 commits behind head on master.

Additional details and impacted files

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