[Feature]: Document extension audio preview/export behavior and author guidance

[crbender]

• I have searched the existing issues

Is your feature request related to a problem?

Yes. Extension authors currently have preview-oriented guidance, but export audio behavior is not clearly documented.

Describe the solution you'd like

Update extension docs to clearly define extension audio behavior in preview and export.

Include:

1. Expected playSound() behavior by mode
2. Export capture semantics
3. Recommended event-driven pattern (cursor:click)
4. Guardrails and practical limits

Describe alternatives you've considered

Keeping implementation details implicit in code only. Rejected, slows contributor adoption and causes repeated confusion.

Additional context

Docs should reflect extension-first roadmap and reduce future issue churn.

Acceptance criteria

1. EXTENSIONS.md documents preview vs export audio semantics.
2. Examples include event-driven click sound guidance.
3. Limitations and warnings are explicit.
4. Release notes entry calls out new export parity behavior.

[crbender]

Tracking under parent issue #572 for sequencing and rollout context.