[FEATURE] Route all stackql downloads via releases.stackql.io and set pystackql/{version} UA #64

Intent

Route every platform (including macOS) through the releases.stackql.io proxy and send a pystackql/{version} User-Agent so pulls are attributable in Analytics Engine.

Current state

• Linux/Windows already use the proxy.
• macOS uses storage.googleapis.com/stackql-public-releases/... (GCS bucket) - bypasses the proxy.
• requests.get sends the default python-requests/x UA.
• Download logic is duplicated in utils/download.py and utils/helpers.py.

Changes (apply to both files, or dedupe to one)

# Darwin URL
- return 'https://storage.googleapis.com/stackql-public-releases/latest/stackql_darwin_multiarch.pkg'
+ return 'https://releases.stackql.io/stackql/latest/stackql_darwin_multiarch.pkg'

# request (set version-stamped UA; import from wherever the package version is defined)
- r = requests.get(url, stream=True)
+ r = requests.get(url, stream=True, headers={"User-Agent": f"pystackql/{__version__}"})

Acceptance

• Linux, Windows, macOS all download from releases.stackql.io
• proxy logs show UA pystackql/<version>
• download logic deduplicated, or both copies updated identically