feat: multi-runtime selection in interactive installer (#1281)

The interactive prompt now accepts comma-separated or space-separated
choices (e.g., "1,4,6" or "1 4 6") to install multiple runtimes in
one go, without needing --all or running the installer multiple times.

- Replaced if/else-if chain with runtimeMap lookup + split parser
- Added hint text: "Select multiple: 1,4,6 or 1 4 6"
- Invalid choices silently filtered, duplicates deduplicated
- Empty input still defaults to Claude Code
- Choice "8" still selects all runtimes

Tests: 10 new tests covering comma/space/mixed separators,
deduplication, invalid input filtering, order preservation,
and source-level assertions.

Closes #1281

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/copilot-install.test.cjs

@@ -150,18 +150,13 @@ describe('Source code integration (Copilot)', () => {
     assert.ok(src.includes('--copilot'), 'help text has --copilot option');
   });
 
-  test('CLI-02: promptRuntime has Copilot as option 5', () => {
-    assert.ok(src.includes("choice === '5'"), 'choice 5 exists');
-    // Verify choice 5 maps to copilot (the line after choice === '5' should reference copilot)
-    const choice5Index = src.indexOf("choice === '5'");
-    const nextLines = src.substring(choice5Index, choice5Index + 100);
-    assert.ok(nextLines.includes('copilot'), 'choice 5 maps to copilot');
+  test('CLI-02: promptRuntime runtimeMap has Copilot as option 5', () => {
+    assert.ok(src.includes("'5': 'copilot'"), 'runtimeMap has 5 -> copilot');
   });
 
-  test('CLI-02: promptRuntime has All option including copilot', () => {
-    // All option callback includes copilot in the runtimes array
-    const allCallbackMatch = src.match(/callback\(\[(['a-z', ]+)\]\)/g);
-    assert.ok(allCallbackMatch && allCallbackMatch.some(m => m.includes('copilot')), 'All option includes copilot');
+  test('CLI-02: promptRuntime allRuntimes array includes copilot', () => {
+    const allMatch = src.match(/const allRuntimes = \[([^\]]+)\]/);
+    assert.ok(allMatch && allMatch[1].includes('copilot'), 'allRuntimes includes copilot');
   });
 
   test('isCopilot variable exists in install function', () => {