Fix dangling config binds: seed configs to a stable host path

Root cause of the Grafana "readdirent /etc/grafana/dashboards: no such file or
directory": the CI runner checks out into an ephemeral act workspace that is
removed after the job, so binding the compose config files straight from it
dangles the mounts in the long-lived containers (verified the act source dir is
emptied after the job). caddy/otelcol/prometheus/tempo read their config once at
startup so they survive, but would break on a restart — same latent bug.

Fix (mirrors ../galaxy-game's $HOME/.galaxy-dev/monitoring): the deploy job seeds
the config dirs to a stable $HOME/.scrabble-deploy and the compose binds them via
${SCRABBLE_CONFIG_DIR:-.} (local runs keep "."). Documented in the compose header,
deploy/README.md and the ci.yaml step.

deploy/README.md

@@ -42,6 +42,14 @@ runs `docker compose up -d --build` on the runner host. Stage 18 (prod) maps the
 **`PROD_`** set the same way. So a Gitea secret named `TEST_POSTGRES_PASSWORD`
 feeds the compose's `POSTGRES_PASSWORD`, etc.
 
+The deploy job also **seeds the config files** (`caddy`, `otelcol`, `prometheus`,
+`tempo`, `grafana`) to a stable host path (`$HOME/.scrabble-deploy`) and sets
+`SCRABBLE_CONFIG_DIR` to it before `up`. The runner's checkout is an ephemeral act
+workspace that is removed after the job — binding config straight from it would
+dangle the mounts in the long-lived containers (Grafana would log
+`no such file or directory`). Locally `SCRABBLE_CONFIG_DIR` defaults to `.`, so the
+compose binds from this directory.
+
 ## Required variables
 
 `docker compose` aborts immediately if any of these is unset (they use `:?`):