fix: make context uploads and downloads space-safe

[madtank]

Summary

• Make default context upload keys unique so repeated image.png uploads do not overwrite prior context entries.
• Add --space-id support for context list/get/delete/download and URL-quote context keys with : and other special characters.
• Let the active CLI profile drive ordinary commands so dev profile/PAT usage is boring; local config still wins when running inside a project.
• Preserve authenticated context file downloads using the API URL and bearer token.

Validation

• python3 -m py_compile ax_cli/config.py ax_cli/client.py ax_cli/context_keys.py ax_cli/commands/context.py ax_cli/commands/upload.py
• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 pytest -q tests/test_context_commands.py tests/test_client.py tests/test_token_cache.py -> 27 passed
• Live dev Team Hub smoke with admin PAT:
  • context list --space-id da183d3f-77ae-4497-be93-b829562cf60a --json returned the 4 expected Team Hub image context entries.
  • context download upload:1775880839429:image.png:df9b1d15-e9c5-4e60-851e-53ea35b4 --space-id da183d3f-77ae-4497-be93-b829562cf60a downloaded a valid 1726x976 PNG.