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>

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 Antoine Van Elstraete
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.