privacy: document temporary chats (incl. as default) and sharpen the honest caveat

- §4: the exact boundary — enclave models protect inference, stored
  history is operator-readable; recommend Temporary Chat + enclave model
  for sensitive material. New subsection on Temporary Chat (per-
  conversation toggle, Default Temporary Chat setting, trade-offs,
  what it does and does not change).
- §8: temporary chats are never retained.
- getting-started: add Temporary Chat to the chat feature list with
  the Settings → Conversations default toggle.
No engineering change; documents what the deployed chat (1.4.5) already
supports. Member request 2026-10-28.

getting-started.md

@@ -53,6 +53,13 @@ with the account you created in step 1.
   there's a tool you'd like available, ask in #general.
 - **Voice** works in both directions: speak to the chat with the microphone
   icon, and have replies read aloud.
+- **Temporary chats** — for conversations you don't want kept on our servers
+  at all. Click the **Temporary Chat** toggle (in the chat header) and the
+  conversation is never saved: it disappears when you leave the page, and
+  nothing lands in your history or our database. To make this your default,
+  open **Settings → Conversations** and turn on **Default Temporary Chat**.
+  For anything sensitive, we recommend a temporary chat — ideally paired with
+  a 🔒 enclave model. See [Privacy](privacy-policy.md#4-what-we-can-see-an-honest-caveat).
 
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