fix: suppress duplicate error output before logging setup

[zanjonke]

Problem

Following #214, the linked-resource validation error (e.g. Plain syntax error: Link ... does not exist.) was printed twice during --dry-run — once in plain white text, then again in red.

Root cause

console.error() in plain2code_console.py both logs the message (logger.error(...)) and rich-prints it in red. On --dry-run (and --status, --full-plain, and early parse errors), the module is parsed before setup_logging() configures any handlers. With no handler attached to the codeplain logger, Python's logging.lastResort handler echoes the logger.error(...) record to stderr as plain white text — so the same message appears twice (white from lastResort, red from rich).

Fix

Attach a NullHandler to the codeplain logger at import time (the standard Python library idiom). This suppresses lastResort so records emitted before real handlers exist aren't echoed to stderr, leaving exactly one output on these pre-logging CLI paths. Once setup_logging() runs, the real handlers work as before.

This also fixes the same latent double-print for any other pre-logging console.error/console.warning call (--status, --full-plain).