[FEATURE] Support custom distribution URL for air-gapped / restricted environments

The Problem

The official install command:

curl -fsSL https://claude.ai/install.sh | bash

redirects to bootstrap.sh on a GCS bucket, which downloads the version info, manifest, and binary — all from the same hardcoded GCS URL:

https://storage.googleapis.com/claude-code-dist-86c565f3-f756-42ad-8dfa-d59b1c096819/claude-code-releases

In corporate or air-gapped environments this URL is blocked, causing installation and upgrades to fail. Setting HTTPS_PROXY can help bootstrap.sh reach GCS, but the downloaded claude binary also has the GCS URL hardcoded — so claude install and auto-update checks fail regardless of proxy settings in the shell.

Suggested Fix

Two small changes would make Claude Code natively installable behind firewalls without any wrapper scripts or sed hacks:

1. Make bootstrap.sh respect a CLAUDE_DIST_URL environment variable

# In bootstrap.sh — replace the hardcoded assignment:
GCS_BUCKET="${CLAUDE_DIST_URL:-https://storage.googleapis.com/claude-code-dist-.../claude-code-releases}"

This lets organizations point the installer at an internal mirror:

export CLAUDE_DIST_URL="https://my.company.com/artifactory/claude-code-releases"
curl -fsSL https://claude.ai/install.sh | bash

2. Make the claude binary respect the same variable

The claude install subcommand and the auto-update mechanism should read CLAUDE_DIST_URL (or a --dist-url flag) to override the hardcoded GCS endpoint. This eliminates the second failure point — the binary calling home to a blocked URL.

Together these changes would make the entire install and update flow work through any HTTP-compatible mirror (Artifactory, Nexus, Cloudsmith, a simple nginx proxy, etc.) with zero patching.