feat(release): RELEASE identity + image tag single-sourcing (#10)

- RELEASE file: single source of release version (0.0.X until declared stable)
- common.sh load_release(): validates version, derives
  MOSAIC_IMAGE_TAG=mosaic-poc-agent:<pi>-r<release> from the pinned pi dep
- compose.yaml: image tag is required env; build/hello/verify call load_release
- verify.sh derives the image name instead of hardcoding it
- package.json version aligned to the same 0.0.X line

Closes #10

compose.yaml

@@ -3,7 +3,7 @@ services:
     build:
       context: .
       dockerfile: Containerfile
-    image: mosaic-poc-agent:0.84.4
+    image: ${MOSAIC_IMAGE_TAG:?MOSAIC_IMAGE_TAG must be set by scripts/load_release (run via scripts/*.sh)}
     user: "1000:1000"
     environment:
       # Resolved from config.json by scripts/common.sh (load_config).