#20997 Gallery: composer-context computer-menu play matches the computer's new accessible name

#20791 put the machine in the composer strip's computer button name ("Computer: This Mac", so the icon-only computer under 460 px stays named). The agent-pane.composer-context computer-menu play still clicked an exact "Computer", found nothing, and test/composer-type-scale.test.ts failed on feat-cmux-next.

Now the play matches /^Computer\b/.

Tests: CMUX_BROWSER_TESTS=1 bun test test/composer-type-scale.test.ts -t composer-context (4 pass, was 1 fail), plus the other browser tests that open composer-context; bun run check clean. No UI change.

🤖 Generated with Claude Code

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Summary by cubic

Fixes the composer-context gallery play so it finds the computer button by its new accessible name. #20791 changed the button's name to include the machine ("Computer: This Mac"), so the exact "Computer" match failed and test/composer-type-scale.test.ts broke on feat-cmux-next. The play now matches /^Computer\b/. No UI change.