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>

docs/OPENBAO-DEPLOYMENT.md

@@ -144,7 +144,7 @@ sleep 30
 docker logs mosaic-openbao-init
 
 # 3. Deploy swarm stack
-IMAGE_TAG=dev ./scripts/deploy-swarm.sh mosaic
+IMAGE_TAG=latest ./scripts/deploy-swarm.sh mosaic
 
 # 4. Verify API connects to OpenBao
 docker service logs mosaic_api | grep -i openbao
@@ -172,7 +172,7 @@ docker logs mosaic-openbao-init
 # OPENBAO_SECRET_ID=...
 
 # 2. Deploy stack (no OpenBao)
-IMAGE_TAG=dev ./scripts/deploy-swarm.sh mosaic
+IMAGE_TAG=latest ./scripts/deploy-swarm.sh mosaic
 
 # 3. Verify API connects to external Vault
 docker service logs mosaic_api | grep -i vault