feat: Cosmos-backed agent memory + invisible per-message timestamps

[bcperry]

Move durable agent state into Azure Cosmos DB and make Cosmos the single source of truth.

• cosmos_memory.py: shared async client, durable chat-history provider, and conversation-index repository (partition /user_id); emulator-aware client (well-known key + no endpoint discovery for localhost)
• user_data.py: per-user Cosmos repositories for custom agents, built-in agent customizations, and user profiles (replaces browser localStorage)
• User-profile memory tools (get/save) read/write Cosmos directly as async function tools bound to user_id; removed the per-session in-memory store and the frontend persistence round-trip
• Prepend the user's local date/time to each user message for model/session temporal context; stripped from the wire form so it stays invisible in the UI
• REST endpoints for custom agents and agent customizations; server-backed frontend hooks with optimistic updates; delete obsolete localStorage hooks
• Terraform: serverless Cosmos account + containers, managed-identity data contributor role, and ENABLE_DEV_COSMOS_ACCESS dev-access toggle
• Tests: emulator-backed integration tests + loop-independent in-memory doubles injected via monkeypatch (no production test seams)