fix(mcp): don’t leave gateway secrets in /tmp after a crash #3596

Summary

pkg/tools/mcp/gateway.go:76-121 writes plaintext secret values to os.CreateTemp files (0600) passed by path to docker mcp gateway run; removal happens only in Stop (gateway.go:58, 63-72). A crash/SIGKILL leaves secrets on disk indefinitely.

Fix

Startup sweep of stale mcp-secrets-* / mcp-config-* files; evaluate passing secrets via stdin/fd instead of a path (keep the file path as a fallback for gateway compatibility).

Acceptance

• Stale secret files cleaned on startup; no plaintext secret persists after abnormal exit (where feasible).