fix(docker): restore base capabilities in sandbox overlay (#523)

Sandbox overlay's cap_add replaces (not merges) the base compose, dropping
SETUID, SETGID, CHOWN. This causes credential copy to fail with Permission
denied when combining sandbox + claude-cli overlays.

Changes:
- Re-include base capabilities in sandbox overlay's cap_add
- Use umask 077 for atomic permission-safe credential copy
- Add ENABLE_CLAUDE_CLI build arg to pre-install Claude CLI in image
- Add runtime warning when credentials mounted but CLI binary missing
- Add WITH_CLAUDE_CLI to Makefile for overlay consistency
- Add security warning comment for sandbox overlay attack surface

docker-compose.claude-cli.yml

@@ -1,10 +1,14 @@
-# Optional overlay: sync Claude CLI credentials from host into container.
+# Optional overlay: install Claude CLI and sync credentials from host into container.
 # Mounts host ~/.claude as read-only; entrypoint copies credentials to the data volume.
+# Adds ENABLE_CLAUDE_CLI build arg to install nodejs + @anthropic-ai/claude-code.
 #
 # Usage:
-#   docker compose -f docker-compose.yml -f docker-compose.postgres.yml -f docker-compose.claude-cli.yml up -d
+#   docker compose -f docker-compose.yml -f docker-compose.postgres.yml -f docker-compose.claude-cli.yml up -d --build
 
 services:
   goclaw:
+    build:
+      args:
+        ENABLE_CLAUDE_CLI: "true"
     volumes:
       - ${HOME}/.claude:/app/.claude-host:ro