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

.env.example

@@ -13,5 +13,9 @@ POSTGRES_PASSWORD=
 # Docker Compose auto-builds this from POSTGRES_USER/PASSWORD/DB.
 # GOCLAW_POSTGRES_DSN=postgres://user:pass@host:5432/dbname?sslmode=disable
 
+# --- Sandbox (only when using docker-compose.sandbox.yml) ---
+# Docker socket GID: 999 on Linux, 0 on Windows/macOS Docker Desktop.
+# DOCKER_GID=0
+
 # --- Debug ---
 # GOCLAW_TRACE_VERBOSE=1