feat(cli): --selector flag scoping lint to subtree

[aram-devdocs]

Summary

Add --selector <css-selector> to scope lint to elements
matching the selector and their descendants.

Crate / area

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

Phase / batch

Part of #20 → Batch 2D (Developer experience — JSON output, selector scoping, init).

Effort: S

PRD references

• PRD §15.4

Acceptance criteria

• CSS selector supported via scraper's matcher.
• Selector filtering applied before rule dispatch.
• CLI integration test.

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