chore: baseline container POC and atomic foundation plan

- Containerized Pi hello-world proof (image mosaic-poc-agent:0.84.4, non-root)
- Four immutable contract fixtures loaded into a generated system prompt
- build/hello/verify/reset scripts with exact-match gating and reset safety
- Documented Pi discovery (v0.84.4, -p mode, --system-prompt, container auth)
- Append-only BUILD-LOG with corrections; deferred layers in LAYERS.md
- Architecture plan: docs/plans/2026-09-02_atomic-mosaic-foundation.md

scripts/common.sh

@@ -0,0 +1,15 @@
+# Shared helpers for the POC host scripts. Not a documented entry point.
+
+MOSAIC_DEV_DIR="/home/jwoltje/.mosaic-dev"
+POC_ROOT_MARKER=".mosaic-poc-root"
+
+# Ensure the runtime state directory exists and carries this project's
+# ownership marker. The marker is what scripts/reset.sh requires before
+# it will delete anything.
+bootstrap_runtime_dir() {
+    if [ ! -d "$MOSAIC_DEV_DIR" ]; then
+        mkdir -p "$MOSAIC_DEV_DIR"
+        echo "bootstrap: created $MOSAIC_DEV_DIR"
+    fi
+    touch "$MOSAIC_DEV_DIR/$POC_ROOT_MARKER"
+}