Remove develop branch references from CI, compose, env, and docs
now that all development uses trunk-based workflow on main.
- CI: remove develop branch filters and dev tag logic
- Compose: default IMAGE_TAG from dev to latest
- Env: update IMAGE_TAG default and comments
- Docs: update branching strategy, PR targets, and image tag docs
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>