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>

.woodpecker/api.yml

@@ -152,12 +152,10 @@ steps:
           DESTINATIONS="--destination git.mosaicstack.dev/mosaic/stack-api:$CI_COMMIT_TAG"
         elif [ "$CI_COMMIT_BRANCH" = "main" ]; then
           DESTINATIONS="--destination git.mosaicstack.dev/mosaic/stack-api:latest"
-        elif [ "$CI_COMMIT_BRANCH" = "develop" ]; then
-          DESTINATIONS="--destination git.mosaicstack.dev/mosaic/stack-api:dev"
         fi
         /kaniko/executor --context . --dockerfile apps/api/Dockerfile --snapshot-mode=redo $DESTINATIONS
     when:
-      - branch: [main, develop]
+      - branch: [main]
         event: [push, manual, tag]
     depends_on:
       - build
@@ -180,7 +178,7 @@ steps:
         elif [ "$$CI_COMMIT_BRANCH" = "main" ]; then
           SCAN_TAG="latest"
         else
-          SCAN_TAG="dev"
+          SCAN_TAG="latest"
         fi
         mkdir -p ~/.docker
         echo "{\"auths\":{\"git.mosaicstack.dev\":{\"username\":\"$$GITEA_USER\",\"password\":\"$$GITEA_TOKEN\"}}}" > ~/.docker/config.json
@@ -188,7 +186,7 @@ steps:
           --ignorefile .trivyignore \
           git.mosaicstack.dev/mosaic/stack-api:$$SCAN_TAG
     when:
-      - branch: [main, develop]
+      - branch: [main]
         event: [push, manual, tag]
     depends_on:
       - docker-build-api
@@ -230,7 +228,7 @@ steps:
         }
         link_package "stack-api"
     when:
-      - branch: [main, develop]
+      - branch: [main]
         event: [push, manual, tag]
     depends_on:
       - security-trivy-api