feat(plugin): ModuleEvent::PluginFileDialog for native file import

Add-ons had no way to request a file picker — only the host's own Open
dialog. PluginFileDialog lets a plugin tool ask the host to open a native
picker; on selection the host dispatches "<command> <path>" back to the
plugin with original case preserved (bypassing the command-line
upper-casing that mangles case-sensitive paths on Linux/macOS). The demo
plugin gains an Import tool exercising it.

Part of the #100 extensibility epic (surfaced in #106).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -722,6 +722,13 @@ pub enum Message {
         tool_id: String,
         event: ModuleEvent,
     },
+    /// Result of a plugin-requested file picker (`ModuleEvent::PluginFileDialog`).
+    /// `path` is `None` when the user cancels. On `Some`, the host dispatches
+    /// `"<command> <path>"` to the plugins with original case preserved.
+    PluginFileDialogResult {
+        command: String,
+        path: Option<std::path::PathBuf>,
+    },
     // ── Application menu ──────────────────────────────────────────────────
     ToggleAppMenu,
     CloseAppMenu,