feat(sessions): optional model for spawn tools and # model completion in composer

spawn_session and spawn_subsession accept an optional model parameter as
an exact provider/model-id (strict matching; unknown specs fail listing
available models; omitting it keeps the inherited model). The chat
composer opens a model completion menu on # and inserts a
#provider/model-id reference into the draft, which agents forward as
the model parameter.

docs/config.md

@@ -273,6 +273,8 @@ A completion notice wakes an idle parent or queues behind in-flight work. Each n
 
 `list_subsessions`, `check_subsession`, and `read_subsession` never yield or change control flow. They are for deliberate inspection or recovery, not completion polling. While a child works, agent-facing `check_subsession` and `read_subsession` withhold partial output and direct the parent to continue independent work or yield at the join point. Output becomes available when the child stops. Included output and transcripts follow a labeled marker and come last, after PI WEB guidance.
 
+Both `spawn_session` and `spawn_subsession` accept an optional `model` parameter, given as an exact `provider/model-id` such as `anthropic/claude-sonnet-4-5`. When set, the new session starts on that model instead of inheriting the dispatching session's model. The match is strict: an unknown or malformed value is rejected with an error. A `#provider/model-id` reference in the prompt (see [Prompt completions](#prompt-completions)) is how users ask for a specific model; agents forward that reference as this parameter.
+
 In **Settings → Session daemon**, these keys are saved on the selected machine. Restart the session daemon on that machine after changing them.
 
 #### `askUser` and `ask_user`
@@ -331,6 +333,14 @@ Shortcut values are keyed by action id. Values are shortcut strings such as `mod
 
 Prefer Settings → Keyboard for editing shortcuts interactively.
 
+## Prompt completions
+
+The chat composer opens completion menus on three trigger characters:
+
+- `/` at the very start of the draft completes session commands.
+- `@` completes file paths: `@` for tracked files, `@ ` (at, then space) or `!@` for all files. Picking one inserts an `@path` reference into the draft, quoted automatically when the path contains spaces.
+- `#` completes the models available to the session, filtered case-insensitively as you type (at most 12 entries). Picking one inserts a `#provider/model-id` reference into the draft, which tells agents the request should run on that model — for example as the `model` parameter of `spawn_session`.
+
 ## Optional completion tools
 
 File and path `@` completions work without extra tools. If `fzf` is available on the PI WEB server's `PATH`, PI WEB uses it to improve completion filtering/ranking; otherwise it falls back to built-in ranking.