docs(config): state data directory behavior positively

Documentation should describe what the software does, not enumerate
what it does not do. Rephrase the data-directory wording around the
positive facts (each directory is independent; a new root starts with
empty state) and drop the "never moves or copies" constructions from
config.md, the config.html env table, and the changeset.

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; selects a new state location without moving existing state</td>
+                      <td>Restart services before changing; existing state stays in the previous data directory</td>
                     </tr>
                     <tr>
                       <td>Session daemon socket</td>

docs/config.md

@@ -141,7 +141,7 @@ 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.
+Each data directory is independent: 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`.