Fix timestamp schema inconsistency between global validation and tool schemas

[Copilot]

The global TimestampSchema only accepted HH:MM:SS format, but the create-clips tool accepted HH:MM:SS,MMM with milliseconds, and the clip-detector prompt examples showed milliseconds. This created a validation mismatch where segments could be created with millisecond timestamps but fail validation elsewhere.

Changes

• Updated global TimestampSchema in schemas/common.mjs to accept optional milliseconds: /^\d{2}:\d{2}:\d{2}(,\d{3})?$/
• Enhanced test coverage to validate both formats, reject invalid millisecond formats (dot notation, incomplete/excessive digits)

Impact

Both formats now validate consistently across:

• Global schema (schemas/common.mjs)
• Tool schema (functions/tools/create-clips.mjs)
• Prompt examples (functions/agents/clip-detector.mjs)
• Processing utilities (functions/utils/video-processing.mjs - already handled both)

// Both formats now valid everywhere
TimestampSchema.parse("00:14:32")      // ✓
TimestampSchema.parse("00:14:32,500")  // ✓ (now consistent)

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