docs(user): explain secret providers and secret requests

docs/README.md

@@ -17,6 +17,7 @@
 - [Use T3 Code as your default browser](./user/default-browser.md)
 - [Devices](./user/devices.md)
 - [Usage and limits](./user/usage.md)
+- [Secrets](./user/secrets.md)
 - [Product usage data](./user/telemetry.md)
 - [Remote access](./user/remote-access.md)
 - [Guest links](./user/guest-links.md)

docs/user/secrets.md

@@ -0,0 +1,58 @@
+# Secrets
+
+T3 Code can read credentials from a secret provider, so the credential stays in your password
+manager and T3 Code saves only a reference to it. Agents can also ask you for a secret in a
+private card instead of in the chat.
+
+The Secrets settings page is in preview and is not in every build yet. Secret requests from agents
+work everywhere.
+
+## Secret providers
+
+1Password is the supported provider. T3 Code reads secrets with the
+[1Password CLI](https://developer.1password.com/docs/cli/) (`op`), so `op` must be installed and
+signed in on the machine that runs the T3 Code server. When you use T3 Code on another machine or
+from your phone, that is the remote machine, not the device in front of you.
+
+To add one, open **Settings → Secrets**, select **Add provider**, and enter a name and the
+1Password account: the shorthand, sign-in address (without `https://`), or ID that
+`op account list` shows. Secret providers belong to the environment you are connected to. Each
+environment keeps its own list.
+
+## Use a secret reference
+
+Credential fields in Settings, such as provider environment variables and credentials, source
+control tokens, usage limit keys, and device host tokens, show a source picker next to the field.
+Pick **Value** to save the credential itself, or pick a secret provider and enter a reference such
+as `op://vault/item/field`. Copy the reference from 1Password with **Copy Secret Reference**.
+
+T3 Code reads the secret when it needs it, for example when a provider starts. Change the
+credential in 1Password and refresh to use the new value.
+
+## When a secret cannot be read
+
+If 1Password is locked or signed out, the provider or feature that needs the secret shows an error
+naming the secret provider. Unlock 1Password on the server's machine, then select **Refresh** in
+**Settings → Providers**. T3 Code never replaces a saved credential it cannot read with another
+login.
+
+## Remove a secret provider
+
+When you remove a secret provider in **Settings → Secrets**, T3 Code lists the fields that read
+from it. Removing it clears those fields, so set them again before you need them. The secrets in
+1Password are not touched.
+
+## When an agent asks for a secret
+
+An agent that needs a token or password shows a secret request card in the thread instead of asking
+you to paste it into the chat. Type or paste the value and select **Save securely**, or select
+**Decline**. You can answer from web, desktop, or mobile.
+
+If the environment has a secret provider, the card also lets you pick it and enter a reference
+instead of the value. T3 Code reads the secret once, when you answer. If 1Password is locked, the
+card says so and the request stays open: unlock it and try again, or enter the value.
+
+The value never appears in the conversation. Most requests keep it from the agent too: T3 Code
+passes it to the tool that needs it. When an agent has to run a command with the value, the card
+says so instead: the value goes into a private file that the agent's commands can read for about 24
+hours.