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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[github-actions]

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

• unit-test — failure — details
• deterministic-gate — failure — details
• coverage — failure — details
• lint — failure — details
• results — failure — details

deterministic-gate

• public_content_change_id_trailer — commit:fe51c19bf783:3 — public contribution contains a Change-Id trailer — Action: remove the value from the public contribution and replace it with a non-sensitive placeholder