feat(FN-3310): add fn update command to CLI reference

Adds a changeset for a new `fn` command and updates the CLI reference documentation with the command details.

Fusion-Task-Id: FN-3310

docs/cli-reference.md

@@ -48,6 +48,28 @@ During fresh initialization, Fusion also installs the bundled `fusion` skill int
 
 ---
 
+## `fn update`
+
+Check for and install the latest `@runfusion/fusion` CLI release from npm.
+
+```bash
+fn update
+fn update --check
+fn update --global
+fn update --json
+fn upgrade
+```
+
+| Option | Description |
+|---|---|
+| `--check` | Check only. Does not install. Exit code `1` when an update is available. |
+| `--global` | Explicitly install globally (`npm install -g @runfusion/fusion@latest`). This is the default behavior. |
+| `--json` | Output machine-readable status: `currentVersion`, `latestVersion`, `updateAvailable`, `updated`. |
+
+`fn upgrade` is an alias for `fn update`.
+
+---
+
 ## `fn research`
 
 Manage persisted research runs from the CLI.