docs: update Reddit auth requirement and fix architecture diagrams across all READMEs

Reddit requires authentication since 2024 (all API requests return 403
without a valid session cookie). Updated all 4 language versions (zh, en,
ja, ko) to reflect this: platform tables, tool choice tables, architecture
diagrams, and FAQ sections.

Also fixed Chinese README architecture diagram: reddit.py backend corrected
from Exa to rdt-cli, wechat.py backend corrected from camoufox+miku to Exa.
Fixed Japanese README reddit.py swap suggestions (removed outdated PRAW/Pushshift).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>