feat(M15): README, README.LLM.md, LICENSE, CHANGELOG, and final cleanup

Complete the M15 documentation and final review milestone:

README.md (French, brief):
- Project description, quick start, usage, deployment link, acknowledgments
- Acknowledgments: pronotepy, icalendar, caldav, slixmpp, pydantic, openai,
  feedparser, beautifulsoup4, and inspiration from pronote-digest (Antoine Coulon)
- Author: Antoine Van Elstraete

README.LLM.md (English, AI agent guide):
- Prerequisites, installation on LXC/VPS (Debian/CentOS)
- Non-secret configuration preparation (all env vars listed)
- Secret variables clearly identified as operator-only
- Pre-deployment checks (check_secrets.py, pip check, dry-run)
- systemd/timer/logrotate installation steps
- Notes for AI agent (do not commit secrets, do not modify existing files)

LICENSE:
- Standard MIT License, copyright Antoine Van Elstraete (2026)

CHANGELOG.md:
- Initial changelog following Keep a Changelog format
- Covers M1 through M14 with milestone summaries
- Gitea Actions noted as planned/optional, not delivered

.gitignore:
- Added FEAT_* pattern for temporary work files

TODO.md:
- M15 items 1-4 checked (README, architecture docs, CHANGELOG+LICENSE, final review)
- M15 item 5 (Gitea Actions) left unchecked (optional, not done)

Co-authored-by: opencode/tech-writer <tech-writer@agents.invalid>

TODO.md

@@ -294,10 +294,10 @@ Mettre en production de façon supervisée (planification, rotation des logs, v
 
 Rédiger la documentation utilisateur et finaliser le projet.
 
-- [ ] Créer `README.md` (installation, configuration `.env`, usage CLI, systemd/docker, limites, RGPD).
-- [ ] Documenter l'architecture (pipeline, modules) en résumé.
-- [ ] Ajouter `CHANGELOG` initial et la licence (MIT).
-- [ ] Revue finale : cohérence avec le guide, aucun secret documenté en clair.
+- [x] Créer `README.md` (installation, configuration `.env`, usage CLI, systemd/docker, limites, RGPD).
+- [x] Documenter l'architecture (pipeline, modules) en résumé.
+- [x] Ajouter `CHANGELOG` initial et la licence (MIT).
+- [x] Revue finale : cohérence avec le guide, aucun secret documenté en clair.
 - [ ] (Optionnel) Configurer Gitea Actions (pytest + bandit + ruff + mypy) pour le déploiement LXC/VPS (Debian/CentOS).
 
 ### Critères d'acceptation