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>

docker/DOCKER-COMPOSE-GUIDE.md

@@ -12,10 +12,10 @@ Pull and run the latest images from the Gitea container registry:
 # Copy environment template
 cp .env.example .env
 
-# Edit .env and set IMAGE_TAG (optional, defaults to 'dev')
-# IMAGE_TAG=dev        # Development images (develop branch)
-# IMAGE_TAG=latest     # Production images (main branch)
+# Edit .env and set IMAGE_TAG (optional, defaults to 'latest')
+# IMAGE_TAG=latest     # Latest images from main branch (default)
 # IMAGE_TAG=658ec077   # Specific commit SHA
+# IMAGE_TAG=v1.0.0     # Specific version tag
 
 # Pull and start services
 docker compose pull
@@ -49,8 +49,7 @@ docker compose -f docker-compose.build.yml up -d --build
 
 The `IMAGE_TAG` environment variable controls which image version to pull:
 
-- `dev` - Latest development build from `develop` branch (default)
-- `latest` - Latest stable build from `main` branch
+- `latest` - Latest build from `main` branch (default)
 - `658ec077` - Specific commit SHA (first 8 characters)
 - `v1.0.0` - Specific version tag
 
@@ -210,7 +209,7 @@ The repository includes three example compose files for common deployment scenar
 ```bash
 # Set in .env
 COMPOSE_PROFILES=full
-IMAGE_TAG=dev
+IMAGE_TAG=latest
 
 # Start all services
 docker compose up -d