Add first-agent tutorial smoke harness

[asim]

Summary

• Add a zero-to-hero CI smoke test that extracts the Your First Agent guide main.go into a clean temporary module, wires it to the local repository, and verifies it with go mod tidy plus go test.
• Include the smoke test in the maintained zero-to-hero harness script.
• Document the maintained smoke command and add the missing inspect step to the Your First Agent walkthrough.

Testing

• go build ./...
• go test ./internal/harness/zero-to-hero-ci -run TestYourFirstAgentTutorialSmoke -count=1
• ./internal/harness/zero-to-hero-ci/run.sh
• golangci-lint run ./...
• go test ./... (fails in this environment because loopback gRPC tests are blocked by the sandbox envoy: "Loopback targets forbidden")

Closes #4128
Closes #4139