feat(vc): add meeting screenshot shortcut

[Ren1104]

Summary

Adds vc +meeting-screenshot so an authorized user or application can save the current final composite JPEG from an ongoing recorded meeting.

Changes

• Register the screenshot shortcut with user and application identity support.
• Validate output paths and JPEG responses; require --overwrite before replacing an existing file.
• Add shortcut documentation, unit tests, and dry-run E2E coverage.

Test Plan

• Remote unit tests were not started: the shared runner timed out before it reached the CLI test lane.
• git diff --check passed.
• Manual CLI verification has not been run in this worktree.

Related Issues

• None

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PR Quality Summary

CI did not complete successfully. Use the failed check links below to decide whether this PR needs a code change or a rerun.

Failed checks

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[CLAassistant]

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0 out of 2 committers have signed the CLA.

❌ zhaoleibd
❌ Ren1104
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