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>

bin/install.js

@@ -4283,6 +4283,17 @@ function promptRuntime(callback) {
     }
   });
 
+  const runtimeMap = {
+    '1': 'claude',
+    '2': 'opencode',
+    '3': 'gemini',
+    '4': 'codex',
+    '5': 'copilot',
+    '6': 'antigravity',
+    '7': 'cursor'
+  };
+  const allRuntimes = ['claude', 'opencode', 'gemini', 'codex', 'copilot', 'antigravity', 'cursor'];
+
   console.log(`  ${yellow}Which runtime(s) would you like to install for?${reset}\n\n  ${cyan}1${reset}) Claude Code  ${dim}(~/.claude)${reset}
   ${cyan}2${reset}) OpenCode     ${dim}(~/.config/opencode)${reset} - open source, free models
   ${cyan}3${reset}) Gemini       ${dim}(~/.gemini)${reset}
@@ -4291,29 +4302,32 @@ function promptRuntime(callback) {
   ${cyan}6${reset}) Antigravity  ${dim}(~/.gemini/antigravity)${reset}
   ${cyan}7${reset}) Cursor       ${dim}(~/.cursor)${reset}
   ${cyan}8${reset}) All
+
+  ${dim}Select multiple: 1,4,6 or 1 4 6${reset}
 `);
 
   rl.question(`  Choice ${dim}[1]${reset}: `, (answer) => {
     answered = true;
     rl.close();
-    const choice = answer.trim() || '1';
-    if (choice === '8') {
-      callback(['claude', 'opencode', 'gemini', 'codex', 'copilot', 'antigravity', 'cursor']);
-    } else if (choice === '7') {
-      callback(['cursor']);
-    } else if (choice === '6') {
-      callback(['antigravity']);
-    } else if (choice === '5') {
-      callback(['copilot']);
-    } else if (choice === '4') {
-      callback(['codex']);
-    } else if (choice === '3') {
-      callback(['gemini']);
-    } else if (choice === '2') {
-      callback(['opencode']);
-    } else {
-      callback(['claude']);
+    const input = answer.trim() || '1';
+
+    // "All" shortcut
+    if (input === '8') {
+      callback(allRuntimes);
+      return;
     }
+
+    // Parse comma-separated, space-separated, or single choice
+    const choices = input.split(/[\s,]+/).filter(Boolean);
+    const selected = [];
+    for (const c of choices) {
+      const runtime = runtimeMap[c];
+      if (runtime && !selected.includes(runtime)) {
+        selected.push(runtime);
+      }
+    }
+
+    callback(selected.length > 0 ? selected : ['claude']);
   });
 }