feat: UX polish — guided launcher + --output json (stretch)

[aptracebloc]

Goal

Round out the experience and keep automation first-class. Stretch — sequence last.

Scope

• Apply the Phase-1 banner/step UX to cluster info (and any other bare-output commands).
• Bare tracebloc (no subcommand) on a TTY → a short guided launcher/menu pointing at cluster info / dataset push; non-TTY → usage (unchanged).
• --output json on dataset push (as version already has) so the pretty TUI never blocks scripting — emit a machine-readable result (summary counts + outcome) and suppress the human screen.

Acceptance criteria

• cluster info renders via the ui banner/steps.
• bare tracebloc on TTY → launcher; non-TTY → usage.
• dataset push --output json emits structured JSON (summary counts, exit status) and suppresses the pretty screen.
• make ci green.

Go concepts we'll cover

• Composing the UX pieces; dual human/machine output paths; encoding/json for the structured result (same pattern as version --output-json).

Notes

• Depends on #PHASE1–#PHASE3. Branch off develop.

Part of the UX/interactive epic.

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