feat: Enable file editing and saving (#252)

## Summary
- Adds file writing capability to Monaco editor in the file viewer
- Implements writeFile API on the server for workspace files
- Integrates save functionality into the file viewer UI with proper
state management

## Bug Fixes (Review Feedback)
- Fixed failed save discarding edits when switching files - now checks
save result and only proceeds if successful
- Fixed refresh overwriting dirty editor state - now prompts for
confirmation before discarding edits
- Fixed save button unable to save empty files - changed check from `if
(content)` to `if (content !== undefined && content !== null)`
- Added agent edit conflict detection - when agent edits file while user
has unsaved changes, shows conflict dialog with Overwrite/Cancel options
- Fixed dialog appearing behind unpinned sidebar - increased alert
dialog z-index to z-100

## Related Issues
- Closes #251

---------

Co-authored-by: Jess Chadwick <jchadwick@gmail.com>

packages/ui/src/components/instance/shell/right-panel/tabs/FilesTab.tsx

@@ -1,7 +1,7 @@
 import { For, Show, Suspense, lazy, type Accessor, type Component, type JSX } from "solid-js"
 import type { FileNode } from "@opencode-ai/sdk/v2/client"
 
-import { RefreshCw } from "lucide-solid"
+import { RefreshCw, Save } from "lucide-solid"
 
 import SplitFilePanel from "../components/SplitFilePanel"
 
@@ -21,13 +21,17 @@ interface FilesTabProps {
   browserSelectedContent: Accessor<string | null>
   browserSelectedLoading: Accessor<boolean>
   browserSelectedError: Accessor<string | null>
+  browserSelectedDirty: Accessor<boolean>
+  browserSelectedSaving: Accessor<boolean>
 
   parentPath: Accessor<string | null>
   scopeKey: Accessor<string>
 
   onLoadEntries: (path: string) => void
-  onOpenFile: (path: string) => void
+  onRequestOpenFile: (path: string) => void
   onRefresh: () => void
+  onSave: (content: string) => void
+  onContentChange: (content: string) => void
 
   listOpen: Accessor<boolean>
   onToggleList: () => void
@@ -38,6 +42,13 @@ interface FilesTabProps {
 }
 
 const FilesTab: Component<FilesTabProps> = (props) => {
+  const handleSave = () => {
+    const content = props.browserSelectedContent()
+    if (content !== undefined && content !== null) {
+      props.onSave(content)
+    }
+  }
+
   const renderContent = (): JSX.Element => {
     const entriesValue = props.browserEntries()
     const entries = entriesValue || []
@@ -86,7 +97,13 @@ const FilesTab: Component<FilesTabProps> = (props) => {
                           </div>
                         }
                       >
-                        <LazyMonacoFileViewer scopeKey={props.scopeKey()} path={payload().path} content={payload().content} />
+                        <LazyMonacoFileViewer
+                          scopeKey={props.scopeKey()}
+                          path={payload().path}
+                          content={payload().content}
+                          onSave={props.onSave}
+                          onContentChange={props.onContentChange}
+                        />
                       </Suspense>
                     )}
                   </Show>
@@ -135,7 +152,7 @@ const FilesTab: Component<FilesTabProps> = (props) => {
                   props.onLoadEntries(item.path)
                   return
                 }
-                props.onOpenFile(item.path)
+                props.onRequestOpenFile(item.path)
               }}
               title={item.path}
             >
@@ -168,14 +185,25 @@ const FilesTab: Component<FilesTabProps> = (props) => {
               </Show>
               <Show when={props.browserError()}>{(err) => <span class="text-error">{err()}</span>}</Show>
             </div>
-
+            <button
+              type="button"
+              class="files-header-icon-button"
+              title={props.t("instanceShell.rightPanel.actions.save") || "Save (Ctrl+S)"}
+              aria-label={props.t("instanceShell.rightPanel.actions.save") || "Save"}
+              disabled={props.browserSelectedSaving() || !props.browserSelectedDirty()}
+              style={{ "margin-inline-start": "auto" }}
+              onClick={handleSave}
+            >
+              <Show when={props.browserSelectedSaving()} fallback={<Save class="h-4 w-4" />}>
+                <RefreshCw class="h-4 w-4 animate-spin" />
+              </Show>
+            </button>
             <button
               type="button"
               class="files-header-icon-button"
               title={props.t("instanceShell.rightPanel.actions.refresh")}
               aria-label={props.t("instanceShell.rightPanel.actions.refresh")}
               disabled={props.browserLoading()}
-              style={{ "margin-inline-start": "auto" }}
               onClick={() => props.onRefresh()}
             >
               <RefreshCw class={`h-4 w-4${props.browserLoading() ? " animate-spin" : ""}`} />
@@ -198,4 +226,4 @@ const FilesTab: Component<FilesTabProps> = (props) => {
   return <>{renderContent()}</>
 }
 
-export default FilesTab
+export default FilesTab