feat(mosaic): migrate install wizard from v0 to v1

Complete port of the ~2000 LOC mosaic install wizard from v0. Migrates
all 27 source files across the prompter, config, platform, template,
runtime, skills, and stages subsystems to v1 ESM/NodeNext conventions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/mosaic/src/runtime/detector.ts

@@ -0,0 +1,82 @@
+import { execSync } from 'node:child_process';
+import { platform } from 'node:os';
+import type { RuntimeName } from '../types.js';
+
+export interface RuntimeInfo {
+  name: RuntimeName;
+  label: string;
+  installed: boolean;
+  path?: string;
+  version?: string;
+  installHint: string;
+}
+
+const RUNTIME_DEFS: Record<
+  RuntimeName,
+  { label: string; command: string; versionFlag: string; installHint: string }
+> = {
+  claude: {
+    label: 'Claude Code',
+    command: 'claude',
+    versionFlag: '--version',
+    installHint: 'npm install -g @anthropic-ai/claude-code',
+  },
+  codex: {
+    label: 'Codex',
+    command: 'codex',
+    versionFlag: '--version',
+    installHint: 'npm install -g @openai/codex',
+  },
+  opencode: {
+    label: 'OpenCode',
+    command: 'opencode',
+    versionFlag: 'version',
+    installHint: 'See https://opencode.ai for install instructions',
+  },
+};
+
+export function detectRuntime(name: RuntimeName): RuntimeInfo {
+  const def = RUNTIME_DEFS[name];
+  const isWindows = platform() === 'win32';
+  const whichCmd = isWindows ? `where ${def.command} 2>nul` : `which ${def.command} 2>/dev/null`;
+
+  try {
+    const pathOutput =
+      execSync(whichCmd, {
+        encoding: 'utf-8',
+        timeout: 5000,
+      })
+        .trim()
+        .split('\n')[0] ?? '';
+
+    let version: string | undefined;
+    try {
+      version = execSync(`${def.command} ${def.versionFlag} 2>/dev/null`, {
+        encoding: 'utf-8',
+        timeout: 5000,
+      }).trim();
+    } catch {
+      // Version detection is optional
+    }
+
+    return {
+      name,
+      label: def.label,
+      installed: true,
+      path: pathOutput,
+      version,
+      installHint: def.installHint,
+    };
+  } catch {
+    return {
+      name,
+      label: def.label,
+      installed: false,
+      installHint: def.installHint,
+    };
+  }
+}
+
+export function getInstallInstructions(name: RuntimeName): string {
+  return RUNTIME_DEFS[name].installHint;
+}