ci: remove the benchmark pull request title scope (#249)

## Description

The CLI's benchmark and scenario commands were removed in #248, but the
pull request title check still accepted `benchmark` as a scope. This
removes it, so new titles can no longer use a scope for a feature that
no longer exists.

## Type of Change

- [x] Code refactoring

## Changes Made

- `.github/workflows/pr-title.yml`: remove `benchmark` from the allowed
scopes.

## Testing

A one-word change to the allowed-scope list, with no other logic
touched. The workflow runs on this PR's own title.

## Additional Notes

`CHANGELOG.md` still lists historical benchmark releases. I left it
alone because it records past releases.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: james-rl

.github/workflows/pr-title.yml

@@ -37,7 +37,7 @@ jobs:
             
             // Define allowed scopes (optional)
             const scopes = [
-              'cli', 'mcp', 'devbox', 'benchmark', 'secret',
+              'cli', 'mcp', 'devbox', 'secret',
               'blueprint', 'storage-object', 'network-policy',
               'main', 'snapshot', 'config', 'auth', 'deps'
             ];