refactor(cli): propagate process cancellation

[pablontiv]

Related to #48 — final process phase.

Summary

• Propagate cancellation through Cobra ExecuteContext and install Interrupt/SIGTERM handling only at the process-owner entry point.
• Make updater staging waits deterministic when cancellation races completion or timeout.
• Preserve Picokit’s bounded, non-context-aware fetch as a documented detached residual after caller cancellation.
• Use nil-safe per-run command/updater dependencies without global composition hooks.
• Synchronize the subprocess signal test through explicit inherited-pipe readiness.

Validation

• just ci — passed (85.4% total coverage).
• go test -race ./cmd/backscroll -count=1 — passed.
• Windows test-binary compile — passed.
• Review feedback addressed; equivalent rebase confirmed by range-diff and stable patch IDs.