[FEATURE]: Support GITHUB_TOKEN in install script (API rate limit + private repos)

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

The install script makes three anonymous GitHub requests:

1. Version detection: curl -s https://api.github.com/repos/anomalyco/opencode/releases/latest
2. Release existence check: curl -sI .../releases/tag/v<ver>
3. Release asset download: curl -L .../releases/download/...

Anonymous calls to api.github.com are rate-limited to 60/hour per IP. On shared CI runners this limit is often exhausted, so the version fetch fails and the install aborts (same root cause as #35120 for github/action.yml). Anonymous requests also make it impossible to install opencode from a private fork or mirror.

Proposal: if $GITHUB_TOKEN is set, send an Authorization: Bearer $GITHUB_TOKEN header on all three curl calls. Behavior is unchanged when the variable is empty.