docs: document Pi extension dialogs and extensionDialogsTimeoutMs

Add the close-out documentation for extension dialog support:

- docs/config.md (+ synchronized config.html): extensionDialogsTimeoutMs
  in the config matrix, reload/restart guidance, the global config
  example, and a new Extension dialogs key-detail section covering the
  unattended-dialog safety valve (default 5 min, 0 = forever, effective
  deadline is the sooner of the extension's own timeout and this knob).
- docs/plugins.md (+ synchronized plugins.html): new Pi extension
  dialogs in PI WEB behavior note for extension authors — confirm/select/
  input render inline in the transcript and resolve with the real answer,
  answers use a dedicated daemon channel (never the prompt queue, so
  tool_call hooks park safely), session_start dialogs are reachable
  during create and open, reload rehydration, first-answer-wins across
  tabs, abort/runtime-replacement settlement, and the reload-mid-startup
  browser-local caveat.
- Add the extension-dialogs changeset (patch) for the release notes.

docs/config.md

@@ -39,7 +39,7 @@ Process restarts depend on the key:
 
 - `host` / `port`: restart the gateway web/API service or process.
 - `maxUploadBytes`: restart both the web/API process and the session daemon on that machine.
-- `agent.command` / `agent.dir` / `spawnSessions` / `subsessions` / `askUser`: restart the session daemon on that machine.
+- `agent.command` / `agent.dir` / `spawnSessions` / `subsessions` / `askUser` / `extensionDialogsTimeoutMs`: restart the session daemon on that machine.
 - `pathAccess`: applies on the next request; existing file views may need a browser refresh.
 - `uploads.defaultFolder`: applies to newly opened Files upload dialogs and new direct drag/drop batches after config/workspace refresh.
 - `plugins`: reload the browser tab after changing PI WEB plugin enablement.
@@ -66,6 +66,7 @@ Process restarts depend on the key:
   "spawnSessions": true,
   "subsessions": false,
   "askUser": true,
+  "extensionDialogsTimeoutMs": 300000,
   "plugins": {
     "workspace-tasks": { "enabled": true },
     "updates": { "enabled": true },
@@ -118,6 +119,7 @@ Rows with JSON key `—` are runtime-only environment variables, not config-file
 | Agent can spawn sessions | `spawnSessions` | `PI_WEB_SPAWN_SESSIONS` | Global/session daemon | Not supported locally | Restart session daemon on that machine |
 | Tracked subsessions (beta) | `subsessions` | `PI_WEB_SUBSESSIONS` | Global/session daemon | Not supported locally; also requires `spawnSessions` | Restart session daemon on that machine |
 | Agent can post question forms | `askUser` | `PI_WEB_ASK_USER` | Global/session daemon | Not supported locally | Restart session daemon on that machine |
+| Extension dialog auto-cancel timeout | `extensionDialogsTimeoutMs` | — | Global/session daemon | Not supported locally | Restart session daemon on that machine |
 | Plugin enablement/settings | `plugins.<id>.enabled`, `plugins.<id>.settings` | — | Global | Not core local config; plugins may read their own project files | Reload browser tab |
 | Keyboard shortcuts | `shortcuts.<actionId>` | — | Global | Not supported locally | Applies after settings save/config refresh |
 | Project config version | `version` | — | Project | Project-local only; must be `1` when present | Next project-config read |
@@ -289,6 +291,14 @@ Sending an ordinary chat message while a form is open voids the form: the card c
 
 Restart the session daemon after changing `askUser` or after upgrading PI WEB to a version that introduces this tool. For the systemd user service, run `systemctl --user restart pi-web-sessiond`.
 
+### Extension dialogs
+
+Pi extensions can ask the user questions from `ctx.ui.confirm()`, `ctx.ui.select()`, and `ctx.ui.input()` — including from `session_start` hooks and in-flight `tool_call` hooks. PI WEB renders these dialogs inline in the session transcript and answers them through a dedicated session-daemon channel, never the prompt queue, so a dialog parked inside a `tool_call` hook cannot deadlock the run. Dialog support is always on; there is no enable flag. See [Pi extension dialogs in PI WEB](https://pi-web.dev/plugins#pi-extension-dialogs) for behavior details and author guidance.
+
+`extensionDialogsTimeoutMs` is the unattended-dialog safety valve: how long the session daemon waits for an answer before settling the dialog with its kind's cancel value (`false` for confirm, `undefined` for select and input). It defaults to `300000` (5 minutes); set it to `0` to wait forever. An extension's own `timeout` option still applies, and the effective deadline is the sooner of the two.
+
+The key is edited directly in the global config file. Restart the session daemon after changing it — for the systemd user service, run `systemctl --user restart pi-web-sessiond`.
+
 ### Plugin config
 
 The `plugins` key is only for PI WEB browser plugin enablement/settings on the machine whose config you are editing. It does not install, remove, or update Pi packages; use **Settings → Pi packages** or Pi's package manager for package operations. In a federated setup, **Settings → PI WEB plugins** and **Settings → Pi packages** both target the currently selected machine, and each panel labels where changes will be saved or run.