chore(agent): replace stdlib log with zap in gRPC interceptor

[r2dedios]

Summary

• Replace the last 3 log.Printf calls in the gRPC LoggingInterceptor with structured zap logging
• Remove the stdlib log import from cmd/agent/agent.go
• The entire codebase now uses zap consistently — no stdlib log remains

Test plan

• make local-build — compiles clean
• make lint — 0 issues
• Verified no remaining log.Print* or "log" imports in cmd/ or internal/

Closes #114