feat(docs): add the persistence guide

[joesaunderson]

Closes #1382.

Summary

Adds a dedicated Persistence guide (docs/guides/persistence.md) covering the full storage story that today only exists as a short section in the hooks guide:

• What to persist: the chat row, the SessionState cursor, and the event log — and why they are three separate things
• A recommended two-table schema for a chat product (chats + chat_events), with meta.id as the dedupe key and an app-owned seq for ordering
• Storing events from the client (onEvent/onFinish) and from a server-side hook, including the sessionId → chatId mapping a hook needs
• Skipping *.appended delta events in storage, and backfilling from the durable stream with stream({ follow: false })
• Loading events back and reconstructing the conversation: initialEvents/initialSession in the browser, and folding defaultMessageReducer() from eve/client into EveMessage[] anywhere else
• Resuming across session expiry/completion while keeping one chat history

Builds on the stable event ids from #1239. Cross-links added from the hooks, continuations, and frontend overview pages, and the page is registered in docs/guides/meta.json.

Verification

• pnpm docs:check — 82 files validated, all snippet import paths resolve, MDX compiles

PR Checklist

• I linked an issue with prior discussion confirming this change is wanted
• I ran the relevant checks from CONTRIBUTING.md
• I added tests and documentation where relevant (docs-only change)
• I added a changeset if this touches the published eve package (docs-only, no changeset)
• DCO sign-off passes for every commit (git commit --signoff)

🤖 Generated with Claude Code

https://claude.ai/code/session_01R9KPb7XDbWjxngtZ93EDeH

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