docs(config): document independent data directories

Changing PI_WEB_DATA_DIR selects where managed state lives but never
moves or copies existing state. Explain how to carry session archives
over manually, and fix the stale "moves managed state location" claim
in the config.html env table.

docs/config.html

@@ -415,7 +415,7 @@
                       <td><code>PI_WEB_DATA_DIR</code></td>
                       <td>Process/env</td>
                       <td>Not supported locally</td>
-                      <td>Restart services before changing; moves managed state location</td>
+                      <td>Restart services before changing; selects a new state location without moving existing state</td>
                     </tr>
                     <tr>
                       <td>Session daemon socket</td>

docs/config.md

@@ -141,6 +141,8 @@ Rows with JSON key `—` are runtime-only environment variables, not config-file
 
 `PI_WEB_DATA_DIR` sets the root for PI WEB-managed runtime state and defaults to `~/.pi-web`. Unless a more specific path override is configured, PI WEB stores its project and machine registries, locally discovered plugins, default session-daemon socket, and session archives beneath this root.
 
+Changing `PI_WEB_DATA_DIR` only selects where state lives; it never moves or copies existing state. Each data directory is independent, so after pointing PI WEB at a new root it starts there with empty registries and no session archives. To carry session archives over, stop PI WEB, then copy `archived-sessions.json` and the `archived-sessions/` directory from the old data directory into the new one before starting it again.
+
 This setting does not change the PI WEB config file selected by `PI_WEB_CONFIG` or Pi-owned state such as the active session files selected by `PI_CODING_AGENT_SESSION_DIR`.
 
 ### External path access