feat(cli): add one-command local demo launcher

[gnanirahulnutakki]

Summary

• add sith launch as the single graphical entry point
• select the native desktop on macOS and the loopback-only UI elsewhere, with --mode auto|desktop|ui
• reuse the bounded in-memory kubeconfig-directory importer and allow explicit-directory bootstrap without a default kubeconfig
• reject UI listener/browser flags in desktop mode instead of silently ignoring them
• lead README first-run guidance with the launcher

Security and operability

• creates no workload and performs no cluster mutation
• adds no external listener, telemetry, credential persistence, or new UI implementation
• preserves the existing desktop in-process origin and UI loopback validation
• keeps directory-import failures sanitized

Verification

• go test -race -count=1 ./internal/cli
• 20 repeated internal/cli runs
• built-binary loopback launch smoke from a bounded self-managed test kubeconfig directory: HTTP 200 and clean interrupt shutdown
• make ci
• make e2e-isolation plus both 50,000-case tenancy fuzzers
• make e2e-helm
• make e2e-oci
• make e2e-kind
• go mod tidy -diff and go mod verify
• make release-check with two reproducible four-platform builds and Hub OCI-layout verification

Closes #315.

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