Support Argo dry-run execution on PRs with diff visibility

[jsbroks]

We want to improve the PR workflow by enabling Argo “dry run” executions when a pull request is opened or updated, and surface the resulting diffs back to users.

Goal

Provide a clear, repeatable way to:

1. Trigger Argo dry runs on PRs
2. Capture the resulting plan/diff
3. Display that diff back in the PR (comment, check, or status)

Scope

Document an example GitHub Actions workflow that:

• Triggers on PR open/update
  • Calls Argo dry-run / plan endpoint
  • Retrieves and formats the diff output
• Define how diffs should be surfaced:
  • PR comment (preferred for visibility)
  • OR GitHub Check output
• Optionally: design a native GitHub bot experience for this flow (not sure how we would implement this. Maybe we need to add some annotations to provide the bot more information

Acceptance Criteria

• A working example workflow exists in a repo
• Opening a PR triggers an Argo dry run
• Diff/plan output is automatically posted back to the PR
• Documentation exists explaining how to set this up