chore: remove Docker support and prune repo redundancies

- Drop Dockerfile, docker-compose.yml, .dockerignore and Docker
  references in deployment/troubleshooting/manual-config docs (EN+ZH)
- Delete .agents/ skills mirror (byte-identical duplicate of skills/;
  not produced by scripts/sync_skills.py and unreferenced)
- Drop unused docs media: install.mp4, run.mp4, install-demo.png,
  run-demo.png (~11M, no references in repo)
- Fold unreleased changelogs into release files: unreleased-opencode
  was already covered by 1.1.0; merge release-automation into 1.2.0;
  drop the Unreleased row from CHANGELOG.md index
- Ignore .DS_Store

Author: bx33661