CI: jobs run in a container, PlatformIO directly in it, the toolchains in a volume

The runner now gives each job a container. The workflow asks for
python:3.12-slim, installs git, a compiler and PlatformIO, and mounts the
roro9stack-pio volume as the cache; the scripts skip their own docker run
when RORO_NO_DOCKER says they're in the build container already. A tag
from before the framework was rebuilt gets the stock framework libraries
back before it builds.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

scripts/_docker.sh

@@ -1,8 +1,10 @@
 # Shared helper: run a command inside the roro9stack build container.
+# With RORO_NO_DOCKER set, the caller is in such a container already (a CI job): the command runs
+# right here, in the checkout.
 IMAGE=roro9stack-build
 ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
 
-docker build -q -t "$IMAGE" "$ROOT/docker" >/dev/null
+[ -n "${RORO_NO_DOCKER:-}" ] || docker build -q -t "$IMAGE" "$ROOT/docker" >/dev/null
 
 # The Debug Console token (ADR 0004): made once, kept with the OTA key, never committed.
 DEBUG_TOKEN_FILE="$HOME/.config/roro9stack/debug-token"
@@ -12,6 +14,10 @@ if [ ! -s "$DEBUG_TOKEN_FILE" ]; then
 fi
 
 run_in_container() {
+    if [ -n "${RORO_NO_DOCKER:-}" ]; then
+        (cd "$ROOT" && RORO_DEBUG_TOKEN="$(cat "$DEBUG_TOKEN_FILE")" "$@")
+        return
+    fi
     docker run --rm \
         -u "$(id -u):$(id -g)" -e HOME=/tmp \
         -e RORO_DEBUG_TOKEN="$(cat "$DEBUG_TOKEN_FILE")" \