TDOC-XXX docs: update AGENTS.md with HTTP implementation and architectural details

- Replace FTP references with HTTP-based crawling (requests/BeautifulSoup)
- Document portal.py module (PortalSession, authentication, metadata fetching)
- Add subdirectory detection and fuzzy meeting-name matching patterns
- Update test mocking from FTP to HTTP (requests.Session)
- Document CLI helper function implementations
- Add Architectural Decisions section (5 key decisions)
- Update file-size guidelines and documentation workflow examples

Addresses 11 documented issues (5 critical, 3 medium, 3 low priority)
from docs/REVIEW_AND_IMPROVEMENTS_AGENTS_MD.md.

See: docs/REVIEW_AND_IMPROVEMENTS_AGENTS_MD.md