feat(tools): implement tactful extraction rework for token efficiency

- Restore precision to read_file with 1-based start_line and end_line for Gemini 3.
- Update tool descriptions to establish extraction hierarchy (rg > shell/sed > read_file).
- Codify 'Be Token-Frugal' mandate in system prompt snippets.
- Refine research workflow to allow context-based validation via search tools.
- Update unit tests and verified build integrity.

packages/core/src/utils/fileUtils.test.ts

@@ -950,6 +950,28 @@ describe('fileUtils', () => {
       expect(result.linesShown).toEqual([6, 10]);
     });
 
+    it('should support startLine and endLine for text files', async () => {
+      const lines = Array.from({ length: 20 }, (_, i) => `Line ${i + 1}`);
+      actualNodeFs.writeFileSync(testTextFilePath, lines.join('\n'));
+
+      const result = await processSingleFileContent(
+        testTextFilePath,
+        tempRootDir,
+        new StandardFileSystemService(),
+        undefined,
+        undefined,
+        5,
+        10,
+      ); // Read lines 5-10 (1-based)
+      const expectedContent = lines.slice(4, 10).join('\n');
+
+      expect(result.llmContent).toBe(expectedContent);
+      expect(result.returnDisplay).toBe('Read lines 5-10 of 20 from test.txt');
+      expect(result.isTruncated).toBe(true);
+      expect(result.originalLineCount).toBe(20);
+      expect(result.linesShown).toEqual([5, 10]);
+    });
+
     it('should identify truncation when reading the end of a file', async () => {
       const lines = Array.from({ length: 20 }, (_, i) => `Line ${i + 1}`);
       actualNodeFs.writeFileSync(testTextFilePath, lines.join('\n'));