feat(cli,core): --viewport flag + viewport multi-run orchestration

[aram-devdocs]

Summary

Add --viewport <name> flag (repeatable) to filter the
configured viewport set. CLI iterates over viewports, reuses
the browser process, and runs rules in parallel across
viewports.

Crate / area

plumb-cli — the plumb binary (only crate allowed stdout/stderr + anyhow)

Phase / batch

Part of #10 → Batch 1B (Integration — real DOMSnapshot + viewport multi-run).

Effort: M

PRD references

• PRD §10.3
• PRD §15.4

Acceptance criteria

• plumb lint <url> --viewport mobile --viewport desktop runs against exactly those two viewports.
• Absent --viewport, every configured viewport runs.
• Browser reused across viewport iterations (one launch per CLI invocation).
• Exit code reflects the worst severity across viewports.

Dependencies

• None (first in batch; batch-level deps gate this).

Review gates

• 02-spec-reviewer
• 03-code-quality-reviewer
• 05-architecture-validator
• 04-test-runner

How to deliver

Dispatch /gh-issue <this-issue-number>. See the root AGENTS.md and the scoped plumb-cli/AGENTS.md for the invariants that apply.

Suggested delivery: gh-issue