fix(ai-chat): stop echoing app draft value in global chat write tool results (#9658)

finishAppDraftWrite returned `item: result.item`, whose `value` is the entire
app draft (every frontend file body and inline runnable). Each write_app_file /
patch_app_file / write_app_runnable therefore re-sent the whole app back to the
model; on a large app a few edits overflow the 200k context window.

This restores #9530 (which removed the echo) — the DB-backed-draft refactor
(#9601) reintroduced it by routing all app writes through this shared helper
with `item:` re-added. Write results now return only `{ success, message }`,
matching the flow write tools. Adds a regression test asserting the value is
not echoed.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/copilot/chat/global/core.test.ts

@@ -783,6 +783,40 @@ describe('global AI tools', () => {
 		expect(getBackendDraft('raw_app', 'u/admin/live_app', { workspace: WORKSPACE })).toBeUndefined()
 	})
 
+	it('does not echo the app value back to the model on write', async () => {
+		const sentinel = 'SENTINEL_DO_NOT_ECHO_DEADBEEF'
+		seedBackendDraft(
+			'raw_app',
+			'',
+			{
+				summary: 'Echo check',
+				files: { '/src/App.tsx': `export default function App() { return '${sentinel}' }` },
+				runnables: {},
+				data: { tables: [] }
+			},
+			{ workspace: WORKSPACE }
+		)
+		UserDraft.setLiveEditorDraft({
+			workspace: WORKSPACE,
+			itemKind: 'raw_app',
+			storagePath: '',
+			effectivePath: 'u/admin/echo_app'
+		})
+
+		const raw = await callGlobalTool('write_app_file', {
+			path: 'u/admin/echo_app',
+			file_path: '/src/New.tsx',
+			content: 'export default function New() { return null }'
+		})
+
+		const parsed = JSON.parse(raw)
+		expect(parsed.success).toBe(true)
+		expect(parsed.item).toBeUndefined()
+		// Neither the pre-existing file body nor the just-written one is resent.
+		expect(raw).not.toContain(sentinel)
+		expect(raw).not.toContain('function New')
+	})
+
 	it('discards a draft without deleting the workspace item', async () => {
 		await callGlobalTool('write_script', {
 			path: 'f/scripts/discard-me',

frontend/src/lib/components/copilot/chat/global/core.ts

@@ -2399,13 +2399,13 @@ function draftWriteFailure(result: DraftPersistResult, ctx: WriteDraftCtx): stri
 function finishAppDraftWrite(
 	result: DraftPersistResult,
 	ctx: WriteDraftCtx,
-	onSaved: (item: WorkspaceItem) => { content: string; message: string }
+	onSaved: () => { content: string; message: string }
 ): string {
 	const failure = draftWriteFailure(result, ctx)
 	if (failure) return failure
-	const { content, message } = onSaved(result.item)
+	const { content, message } = onSaved()
 	ctx.toolCallbacks.setToolStatus(ctx.toolId, { content, result: 'Saved as draft' })
-	return JSON.stringify({ success: true, message, item: result.item }, null, 2)
+	return JSON.stringify({ success: true, message }, null, 2)
 }
 
 function finishDraftWrite(