Fix saved config profiles with env overrides

[willccbb]

Summary

• Make prime config use <profile> fail when active credential/user/API URL env vars or ambient PRIME_CONTEXT would mask the selected saved profile.
• Keep one-shot --context working for config use and restore context state after the command.
• Keep existing active saved-profile syncing, but write persisted config values instead of env-overridden values.

Testing

• uv run ruff check packages/prime/src/prime_cli/core/config.py packages/prime/src/prime_cli/commands/config.py packages/prime/src/prime_cli/main.py packages/prime/tests/test_config_profiles.py packages/prime/tests/test_config_delete.py
• uv run ruff format --check packages/prime/src/prime_cli/core/config.py packages/prime/src/prime_cli/commands/config.py packages/prime/src/prime_cli/main.py packages/prime/tests/test_config_profiles.py packages/prime/tests/test_config_delete.py
• uv run pytest packages/prime/tests/test_config_profiles.py packages/prime/tests/test_config_delete.py packages/prime/tests/test_switch.py -q

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