feat: add diagnostics tool for live command output monitoring

- Implemented diagnostics functionality to start, stop, and monitor background check commands.
- Created a DiagnosticsPanel for real-time output display in the UI.
- Added support for auto-detecting default diagnostics commands based on project configuration.
- Introduced run_checks tool to execute project verification commands and report results.
- Enhanced tools-extra with functions to parse AGENTS.md and package.json for check commands.
- Added diff review functionality to visualize changes made in the last turn.
- Implemented tests for diagnostics and run_checks functionalities to ensure reliability.

test/diagnostics.test.ts

@@ -0,0 +1,106 @@
+import { afterEach, beforeEach, describe, expect, it } from 'bun:test';
+import { diagStart, diagStop, diagStatus, bootDiagnostics, defaultDiagnosticsCommand, shutdownDiagnostics } from '../src/diagnostics';
+
+// Diagnostics spawns real processes, so each test gets a fresh state (the module
+// is a singleton — one diagnostics command at a time).
+describe('diagnostics module', () => {
+  afterEach(() => {
+    shutdownDiagnostics();
+  });
+
+  it('starts, shows running status, and stops a command', async () => {
+    const started = diagStart('node -e "setInterval(()=>{}, 1000)"');
+    expect(started.started).toBe(true);
+
+    const status = diagStatus();
+    expect(status.running).toBe(true);
+    expect(status.command).toContain('setInterval');
+
+    const stopped = diagStop();
+    expect(stopped.stopped).toBe(true);
+    expect(diagStatus().running).toBe(false);
+  });
+
+  it('reports exit code once the command finishes', async () => {
+    diagStart('node -e "process.exit(3)"');
+    // Wait for the process to actually exit.
+    await Bun.sleep(300);
+    const status = diagStatus();
+    expect(status.running).toBe(false);
+    expect(status.exit).toBe(3);
+  });
+
+  it('captures output into the tail', async () => {
+    diagStart('node -e "console.log(\'hello-diag\')"');
+    await Bun.sleep(300);
+    const status = diagStatus();
+    expect(status.tail).toContain('hello-diag');
+    expect(status.exit).toBe(0);
+  });
+
+  it('diagStop with nothing running is a no-op', () => {
+    expect(diagStop().stopped).toBe(false);
+    expect(diagStatus().running).toBe(false);
+  });
+
+  it('starting replaces a running command and kills the old one', async () => {
+    const oldHandle = diagStart('node -e "setInterval(()=>{}, 1000)"');
+    expect(oldHandle.started).toBe(true);
+    const second = diagStart('node -e "console.log(\'second\')"');
+    expect(second.started).toBe(true);
+    const status = diagStatus();
+    expect(status.command).toContain('second');
+    // old process is gone; status.command reflects the newest start
+    shutdownDiagnostics();
+  });
+
+  it('bootDiagnostics ignores empty config and starts a real one', async () => {
+    bootDiagnostics(undefined);
+    expect(diagStatus().running).toBe(false);
+    bootDiagnostics('node -e "setInterval(()=>{}, 1000)"');
+    expect(diagStatus().running).toBe(true);
+  });
+
+  it('bootDiagnostics never throws on a bad command — keeps boot clean', async () => {
+    expect(() => bootDiagnostics('')).not.toThrow();
+    // A command that cannot spawn (bad binary) still must not throw synchronously.
+    expect(() => bootDiagnostics('/nonexistent/binary')).not.toThrow();
+    await Bun.sleep(50);
+    shutdownDiagnostics();
+  });
+});
+
+describe('defaultDiagnosticsCommand — auto-detection', () => {
+  const tmp = `${Bun.env['TMPDIR'] ?? '/tmp'}/diag-default-${process.pid}-${Math.random().toString(36).slice(2)}`;
+  let savedCwd: string;
+  const { mkdirSync, rmSync, writeFileSync } = require('node:fs') as typeof import('node:fs');
+
+  beforeEach(() => {
+    savedCwd = process.cwd();
+    rmSync(tmp, { recursive: true, force: true });
+    mkdirSync(tmp, { recursive: true });
+    process.chdir(tmp);
+  });
+
+  afterEach(() => {
+    process.chdir(savedCwd);
+    diagStop();
+  });
+
+  it('picks tsc --watch for a bun+ts project', async () => {
+    writeFileSync('bun.lock', '');
+    writeFileSync('package.json', '{}');
+    writeFileSync('tsconfig.json', '{}');
+    expect(await defaultDiagnosticsCommand(process.cwd())).toBe('bun run typecheck --watch');
+  });
+
+  it('falls back to bun test --watch without tsconfig', async () => {
+    writeFileSync('bun.lock', '');
+    writeFileSync('package.json', '{}');
+    expect(await defaultDiagnosticsCommand(process.cwd())).toBe('bun test --watch');
+  });
+
+  it('returns undefined for an unknown project', async () => {
+    expect(await defaultDiagnosticsCommand(process.cwd())).toBeUndefined();
+  });
+});