feat: gate tarball install behind --beta=tarball flag (#2482)

* feat: gate tarball install behind --beta=tarball flag

Tarball install is not yet reliable enough to be the default.
Move it behind an opt-in --beta=tarball flag so users can test it
explicitly while live install remains the default path.

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

* feat: support multiple --beta flags (repeatable)

Parse all --beta flags from args in a loop, collecting them into a
comma-separated SPAWN_BETA env var. Consumers check for their feature
with Set.has() so multiple beta features can be active simultaneously.

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

* refactor: replace for(;;) loop with extractAllFlagValues helper

Cleaner approach: a dedicated helper mutates args in place and returns
all values for a repeatable flag, replacing the infinite loop pattern.

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

---------

Co-authored-by: lab <6723574+louisgv@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@openrouter/spawn",
-  "version": "0.16.5",
+  "version": "0.16.6",
   "type": "module",
   "bin": {
     "spawn": "cli.js"