Document agent demo in first-agent on-ramp

[asim]

Summary:

• Add micro agent demo to the README and website first-agent on-ramp.
• Link the command from the no-secret transcript and docs index.
• Extend first-agent docs tests to keep the command visible.

Testing:

• go test ./internal/harness/zero-to-hero-ci -run 'TestFirstAgentWayfindingDocs|TestNoSecretFirstAgentTranscript' -count=1\n- go build ./...\n- go test ./... (fails in this environment: loopback targets forbidden in grpc transport/client tests)\n- golangci-lint run ./...\n\nCloses Document micro agent demo in the first-agent on-ramp #4041\nCloses Loop: build increment #81 #4043