[codex] Audit content category and UGC exposure

[jjoonleo]

Closes #460.

Part of parent release track #466.

Summary

• Added a [Release] Audit content category and UGC exposure #460 implementation plan under plans/.
• Documented the current release audit result in docs/Release-Checklist.md: no UGC is exposed to other users.
• Documented that the checked source does not implement gambling, betting, regulated finance, trading, lending, crypto, medical, adult, or other restricted-category flows.
• Added re-audit guidance for future shared/public content surfaces and moderation/report/block requirements.

Verification

• git diff --check
• Source audit searches over lib, docs, README.md, and pubspec.yaml for UGC, sharing, moderation, and restricted-category terms.
• Reviewed endpoint/data-source surfaces for authentication, schedules, preparations, notification, alarm settings/devices/windows/status, and feedback.

Unblocks

• No report/block/moderation sub-issues are required for the current release audit. The [Release Track] Store listing and content #466 store-listing/content track can continue with the remaining ordered release items.