Copilot Fix(CI Failure): Bump pinned copilot-version default to 1.0.83

[github-actions]

The scheduled Test Copilot CLI workflow's version-pin-drift job failed because action.yml's pinned copilot-version default (1.0.80) had drifted behind the @github/copilot@latest npm release (1.0.83).

That job exists specifically to catch this: https://github.com/austenstone/copilot-cli/actions/runs/33951644747/job/93169598879

Why did the CLI drift so far behind while the pin check kept passing? Turns out the pin isn't the only thing that ages — apparently so does my sense of urgency about updating it. 😅

💥 Error Log

##[warning]copilot-version default is pinned to 1.0.80 but @latest is 1.0.83
##[error]Process completed with exit code 1.

🕵️‍♂️ Diagnosis

This is not a transient failure or code bug — it's the intended behavior of the version-pin-drift job (added in #81), which runs on a schedule and intentionally fails when the pinned default copilot-version in action.yml no longer matches the latest @github/copilot package on npm. The last CLI release bumped npm latest to 1.0.83, while action.yml was still pinned to 1.0.80.

🛠️ Proposed Fix

• Bump the copilot-version default in action.yml from 1.0.80 to 1.0.83.
• Update the matching example values/descriptions in README.md (options table and the "Pinning the Copilot CLI version" examples) to 1.0.83.
• Left the illustrative shell comment in action.yml and the >=1.0.80 floor constraint in .github/workflows/test-copilot.yml unchanged, since both remain valid regardless of the current pinned default.

Validated action.yml and .github/workflows/test-copilot.yml still parse as valid YAML after the edit.