local-dev: prune broken engines on rebuild + document one-time bake

`make rebuild` runs `compose build --no-cache backend gateway` plus
a fresh `up -d --wait`. It must therefore also reap any engine
container whose bind-mount source went away during host downtime,
otherwise the new backend image boots into a stack with the same
orphan that triggered the heal flow in the first place.

Also extend the troubleshooting note: pulling the heal-cycle fix
requires one explicit `make rebuild` so the backend image picks up
the pre-bootstrap reconciler tick. Without that, `make up` runs
the new Makefile target but the legacy backend cannot follow
through, and the developer is left staring at a `cancelled`
sandbox with no running replacement.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tools/local-dev/Makefile

@@ -36,7 +36,7 @@ help:
 up: build-engine prune-broken-engines
 	$(COMPOSE) up -d --wait
 
-rebuild: build-engine
+rebuild: build-engine prune-broken-engines
 	$(COMPOSE) build --no-cache backend gateway
 	$(COMPOSE) up -d --wait
 

tools/local-dev/README.md

@@ -239,6 +239,19 @@ make status         docker compose ps
   the cancelled tile and provisions a fresh sandbox with a brand
   new state directory. To run the cleanup by hand without restarting
   the rest of the stack, `make prune-broken-engines`.
+
+  The cycle relies on the backend image carrying the pre-bootstrap
+  reconciler tick (`backend/cmd/backend/main.go`). `make up` reuses
+  the cached image, so after pulling this commit the first time you
+  must `make rebuild` once to bake the fix in. Future `make up`
+  cycles will heal in one shot.
+
+  If after the heal cycle the lobby still shows only a `cancelled`
+  sandbox tile and no running game, the running backend image
+  predates the pre-bootstrap reconciler tick — the periodic ticker
+  cancels the orphan after bootstrap has already returned, leaving
+  the lobby in the half-baked state. `make rebuild` recreates the
+  image and then `make up` lands a fresh sandbox.
 - **`make up` reports a build error mentioning `pkg/cronutil`** —
   upstream module list drifted; copy any new `pkg/<name>/` line into
   the local-dev `backend.Dockerfile` / `gateway.Dockerfile` to match