chore: switch from develop/dev to main/latest image tags

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>

.env.example

@@ -215,11 +215,9 @@ NODE_ENV=development
 # Used by docker-compose.yml (pulls images) and docker-swarm.yml
 # For local builds, use docker-compose.build.yml instead
 # Options:
-#   - dev: Pull development images from registry (default, built from develop branch)
-#   - latest: Pull latest stable images from registry (built from main branch)
-#   - <commit-sha>: Use specific commit SHA tag (e.g., 658ec077)
+#   - latest: Pull latest images from registry (default, built from main branch)
 #   - <version>: Use specific version tag (e.g., v1.0.0)
-IMAGE_TAG=dev
+IMAGE_TAG=latest
 
 # ======================
 # Docker Compose Profiles