feat(cmd): support --format json in mode show

[SShogun]

Hey maintainers, i'm new to open source - please be easy on me & guide me if i'm wrong

What this changes
Adds --format text|json (defaulting to text) to pikopod mode show <sandbox>. Resolves #84

Why
Previously, mode show only printed human-readable summaries, making it difficult for scripts and CI pipelines to inspect the active mode without directly querying the control plane API.

With --format json, users can now retrieve the structured mode specification directly from the CLI. Unsupported formats are rejected with an actionable error message.

No new test fixtures were needed; the regression test uses the repository's existing test infrastructure.

Testing

• Verified RED → GREEN regression testing against upstream main.
• Confirmed JSON output for armed and unarmed modes.
• Verified invalid format handling.
• CGO_ENABLED=0 go build ./cmd/pikopod passes.

Checklist

• go test ./... -count=1 passes
• gofmt -l cmd internal is empty and go vet ./... is clean
• No new cgo dependency (the release artifact is a static binary on every platform)
• Error message strings are unchanged, or the change is deliberate and called out above
• New test fixtures have an entry in testdata/parity/importer/specs/SOURCES.md
• No provider payloads, keys, tokens, or customer data in the diff or in test fixtures

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