fix(studio): support standard credential file

[yaozheng-fang]

Summary

• resolve Studio deployment credentials from explicit options, environment variables, or the default profile in ~/.volc/credentials
• preserve environment-variable precedence over the credential file
• return a concise CLI error when no complete credential pair is available
• document the supported credential sources

Tests

• PYTHONPATH=$PWD pytest -q tests/cli/test_studio_deploy_target.py
• ruff check veadk/cli/cli_frontend.py tests/cli/test_studio_deploy_target.py
• ruff format --check veadk/cli/cli_frontend.py tests/cli/test_studio_deploy_target.py
• Markdown lint for the added documentation blocks
• git diff --check