fix: use host bind mount for /data/repos so tool containers can access mounted repos

The API container used a named Docker volume (repo_data:/data/repos) for
storing repositories. When creating tool instances with direct mount mode,
the API told Docker to bind-mount /data/repos/<repo>:/workspace into the
tool container. But the Docker daemon resolves bind-mount paths on the HOST
filesystem, not inside the API container. Since the host had no /data/repos
(the repos only existed inside the named volume), tool containers mounted
empty directories.

Changed both compose files to use a host bind mount (/data/repos:/data/repos)
instead of a named volume. This ensures:
- The API container and tool containers both see the same /data/repos path
- Bind mounts from /data/repos into tool containers work correctly

For existing installations: repos previously stored in the repo_data named
volume should be copied to /data/repos on the host before restarting the
stack.

Quality gates: compose file syntax valid

docker-compose.yml

@@ -57,7 +57,7 @@ services:
       REPO_BASE_PATH: /data/repos
       INSTANCE_BASE_PATH: /data/instances
     volumes:
-      - repo_data:/data/repos
+      - /data/repos:/data/repos
       - /data/instances:/data/instances
     ports:
       - "8000:8000"
@@ -91,7 +91,6 @@ services:
 volumes:
   postgres_data:
   redis_data:
-  repo_data:
 
 networks:
   backend: