prompt: inject full ignore-aware file list at boot (gitignore-respected, capped 5000)

walk() at boot -> Session.workspaceFiles -> systemPrompt Environment
section + subagent system prompts. read_many_files still on-demand,
but the agent now sees the tree without a tool call.

test/prompt-workspace.test.ts

@@ -0,0 +1,19 @@
+import { expect, test } from 'bun:test';
+import { systemPrompt } from '../src/prompt';
+
+test('workspaceFiles are injected gitignore-style when provided', () => {
+  const prompt = systemPrompt({ cwd: '/repo', workspaceFiles: ['src/app.ts', 'README.md'] });
+  expect(prompt).toContain('Workspace files (2, gitignore-respected');
+  expect(prompt).toContain('src/app.ts');
+  expect(prompt).toContain('README.md');
+});
+
+test('workspaceFiles omitted when empty', () => {
+  const prompt = systemPrompt({ cwd: '/repo', workspaceFiles: [] });
+  expect(prompt).not.toContain('Workspace files');
+});
+
+test('workspaceFiles undefined leaves prompt unchanged', () => {
+  const prompt = systemPrompt({ cwd: '/repo' });
+  expect(prompt).not.toContain('Workspace files');
+});