Make agent preflight failures actionable

[asim]

Summary

• Add Fix and Next guidance for first-agent preflight failures across Go, CLI, provider-key, and gateway-port checks.
• Detect Go versions older than 1.24 and point users to the first-agent and debugging guides.
• Update first-agent docs to describe the actionable preflight behavior.

Testing

• go build ./...
• go test ./cmd/micro/cli/agent -run 'Test(RunAgentPreflight|GoVersionAtLeast|FirstLine)' -count=1 -timeout=60s\n- go test ./cmd/micro -run TestFirstAgentWalkthroughCLIBoundaries -count=1\n- go test ./... (fails in this environment because loopback gRPC targets are blocked by envoy: client/grpc and transport/grpc)\n- golangci-lint run ./...\n\nCloses Make first-agent preflight failures actionable #3841\nCloses Loop: build increment #46 #3850