Fix standalone chat creation and notifications

[jln13x]

Summary

• open new standalone chats immediately as local drafts and materialize them on first send
• include standalone chats in desktop completion feedback
• publish standalone chat completion and attention state to mobile relay clients under the Chats group
• document the fork-specific behavior and add regression coverage

Validation

• vp test apps/web/src/composerDraftStore.test.ts apps/web/src/interactionSounds.test.ts apps/server/src/relay/AgentAwarenessRelay.test.ts
• vp run --filter @t3tools/web typecheck
• vp run --filter './apps/server' typecheck
• vp check