feat: prioritize batched reads for turn efficiency

Move read_many_files to core tool set so it is always offered. Strengthen
system-prompt guidance: read_file now redirects to read_many_files for
multiple files, read_many_files is framed as the primary reading tool with
an explicit batch range (2-20). Add a 'How to work' rule on read
efficiency, and a read-batching instruction in the deep agent variant.
861 tests pass; build clean.

test/prompt.test.ts

@@ -10,6 +10,18 @@ test('every documented tool has usable guidance', () => {
   }
 });
 
+test('read guidance steers the model to batch reads for efficiency', () => {
+  const readMany = TOOL_DOCS.find((d) => d.name === 'read_many_files');
+  const readOne = TOOL_DOCS.find((d) => d.name === 'read_file');
+  expect(readMany?.line).toContain('batch');
+  expect(readMany?.line).toContain('read_file');
+  expect(readOne?.line).toContain('read_many_files');
+  // The "How to work" block also instructs batched reading.
+  const prompt = systemPrompt({ cwd: '/repo', availableTools: ['read_file', 'read_many_files'] });
+  expect(prompt).toContain('Read efficiently');
+  expect(prompt).toContain('read_many_files');
+});
+
 test('only the offered tools are described', () => {
   const rendered = renderTools(['read_file', 'grep']);
   expect(rendered).toContain('read_file');

test/session-features.test.ts

@@ -307,6 +307,8 @@ test('session tools survive tool-set gating, since they are not part of that bud
 
 test('toolSetOf names the set a tool came from, and nothing for a session tool', () => {
   expect(toolSetOf('read_file')).toBe('core');
+  // Batched reading is core so it is always offered — efficiency, not an extra set.
+  expect(toolSetOf('read_many_files')).toBe('core');
   expect(toolSetOf('multi_edit')).toBe('edit-plus');
   expect(toolSetOf('git_log')).toBe('git');
   expect(toolSetOf('todo_write')).toBeUndefined();