feat(mcp): add read-only server mode

[ChaoJiCaiNiao3]

Summary

• add --mcp-read-only for MCP server mode
• expose only a restricted kubectl tool; omit bash, custom tools, and external MCP tools
• revalidate the actual command immediately before execution instead of trusting client-supplied modifies_resource
• fail closed for mutations (including dry-run), kubeconfig changes, ambiguous or compound shell commands, redirection, background execution, environment overrides, nested commands, and lookalike executables
• document Kubernetes RBAC and HTTP authentication requirements

Testing

• go test ./...
• go vet ./...
• all current presubmit scripts: build, vet, autogen, format, Go modules, and mocks
• 14 table-driven read-only boundary cases plus MCP tool-isolation and flag-validation tests

Fixes #508

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