chore: Update MR review instructions YAML

Brendan Lynchrequested to merge
bl-update-mr-review-instructions into main

Description

Aligns the GitLab Duo Code Review custom instructions (.gitlab/duo/mr-review-instructions.yaml) with the GitLab CLI (glab) documentation style guide.

The rules had drifted from the style guide, including one direct contradiction that would cause GitLab Duo to grade MRs against stale guidance.

Changes to the Command documentation block and a new Deprecations block:

  • Fixed boolean flag defaults (was a direct conflict). The old rule said to "Always state the default value for boolean flags: (default false)." The style guide says the opposite: pflag auto-appends (default true) for true-default booleans (don't add it manually), and no annotation is required for false-default booleans.
  • Added positional-argument usage-line conventions (<arg>, [<arg>], <id | branch | url>, [<id | branch>]), which the style guide specifies but the YAML omitted.
  • Added standard metavars for common value types (username, string, int).
  • Added a Deprecations section covering MarkDeprecated message format (lowercase sentence continuation, period-terminated, "use --x instead.", avoid now/currently) and the procedure for deprecating environment variables in root.go.

Related issues and merge requests

Documentation

If this MR adds or updates CLI command documentation, follow the conventions in the GitLab CLI (glab) documentation style guide, run make gen-docs, and commit the updated files in docs/source/.

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