feat: add QQ Agent Mail source adapter

[LearnPrompt]

Summary

• add QQ Agent Mail CLI provider for metadata-only email digest ingestion via agently-cli message +list
• allow email digest metadata to join the main Radar as standalone agentmail source only when EMAIL_DIGEST_INCLUDE_IN_RADAR=1
• keep legacy AgentMail API mode and update docs, workflow env, examples, and Skill instructions

Safety

• default off unless EMAIL_DIGEST_ENABLED=1
• no message +read, no raw/body/html ingestion in the public pipeline
• no real inbox addresses, message ids, bodies, or secrets committed

Verification

• /opt/homebrew/bin/python3.12 -m py_compile scripts/update_news.py
• /Users/carl/.local/bin/uv run --python /opt/homebrew/bin/python3.12 --with-requirements requirements-dev.txt pytest -q tests/test_topic_filter.py -k agentmail
• /Users/carl/.local/bin/uv run --python /opt/homebrew/bin/python3.12 --with-requirements requirements-dev.txt pytest -q -> 235 passed
• git diff --check
• python3 "${CODEX_HOME:-$HOME/.codex}/skills/.system/skill-creator/scripts/quick_validate.py" skills/ai-news-radar
• live smoke: agently-cli --version -> 1.0.11
• live smoke: EMAIL_DIGEST_ENABLED=1 AGENTMAIL_PROVIDER=agently_cli AGENTMAIL_LIMIT=5 AGENTMAIL_ALLOWED_SENDER_DOMAINS=tldrnewsletter.com,daily.therundown.ai python3 - <<'PY' ... -> status ok, provider agently_cli, item_count 2