fix: use stdlib logging instead of print (stderr)

[Jah-yee]

Maintainer feedback in #72: 'use a proper logger instead of changing print everywhere'.

This PR replaces all print() calls with Python stdlib logging, configured to write to stderr via basicConfig. This:

• Satisfies the maintainer's request for a 'proper logger'
• Preserves MCP stdio JSON-RPC framing (only log messages go to stderr, tool data stays on stdout)
• Uses INFO level for process steps, ERROR level for failures

Changes:

• Add import logging and logger = logging.getLogger(__name__)
• Configure logging.basicConfig(level=INFO, stream=sys.stderr, format='%(message)s')
• Replace all print() calls with logger.info() / logger.error()
• Remove the file=sys.stderr from all calls (handled by basicConfig)

Closes #72.

[Jah-yee]

Supersedes Jah-yee/mcp-google-sheets#72. Changes: replaced print→stderr approach with stdlib logging (per maintainer request), still writes to stderr to preserve MCP stdio framing.

[xing5]

Closing as superseded by #80. #80 keeps the stdio logging fix focused, configures logging only from main(), adds stdout-safety regression tests, and avoids broader observability changes in the same patch.