Add canonical PerkOS Chat client

[JulioMCruz]

What changed

• adds the reusable browser WebSocket client for chat.perkos.xyz
• adds a Firestore conversation-metadata repository that cannot accept message bodies
• supports independent threads, realtime messages, acknowledgements and canonical history paging
• exports shared conversation identity helpers and lifecycle operations
• prepares version 0.3.0 for consumption by PerkOS App and Desktop

Why

PerkOS Desktop still sends local window history to /assistant/chat, so it cannot share the same agent conversations as PerkOS App. The canonical protocol and metadata lifecycle belong in Shared Client instead of being copied into each frontend.

Impact

This is the service/client foundation for JOURNEY-CHAT-002. No frontend is switched by this PR alone and no production deployment is included.

Validation

• npm test — 26 tests passed
• npm run typecheck
• npm run build