Copilot Fix(CI Failure): Bump pinned copilot-version default to 1.0.81

[github-actions]

The version-pin-drift job in Test Copilot CLI started failing because a new @github/copilot release landed on npm.

Version pins age like milk, not wine — this one just expired. 🥛

https://github.com/austenstone/copilot-cli/actions/runs/33163630133/job/93016726841#step:3:1

💥 Error Log

##[warning]copilot-version default is pinned to 1.0.80 but @latest is 1.0.81
##[error]Process completed with exit code 1.

🕵️‍♂️ Diagnosis

The version-pin-drift job (added in #81) compares the copilot-version default pinned in action.yml against @github/copilot@latest on the npm registry, and intentionally fails on schedule runs when they diverge (test "schedule" != "schedule"). npm now reports 1.0.81 as latest, while action.yml was still pinned to 1.0.80 — a genuine drift, not a transient/CI issue.

🛠️ Proposed Fix

Bump the pinned default from 1.0.80 to 1.0.81 in:

• action.yml (inputs.copilot-version.default)
• README.md (docs table default column and the "exact pin (default)" example)

No other jobs reference the old exact value (config-legacy intentionally pins an unrelated legacy version 1.0.34, and version-spec uses a floor range >=1.0.80 that remains valid).

Verified locally that the drift check now passes:

$ python3 -c "import yaml;print(yaml.safe_load(open('action.yml'))['inputs']['copilot-version']['default'])"
1.0.81
$ npm view --registry=https://registry.npmjs.org '@github/copilot@latest' version
1.0.81