feat(mcp): lint_url accepts real URLs (not just plumb-fake://)

[aram-devdocs]

Summary

Graduate lint_url to real http(s) URLs via the real
ChromiumDriver from Phase 1. Keep plumb-fake:// for tests.

Crate / area

plumb-mcp — rmcp-based stdio MCP server

Phase / batch

Part of #34 → Batch 3B (lint_url graduation + browser warmth).

Effort: M

PRD references

• PRD §14.2

Acceptance criteria

• Real URL produces violations from real rules (Phase 1/2 rules).
• Typed errors for Chromium-not-found / unsupported version.
• Protocol test: call against a local fixture HTML via http://localhost:<N>/fixture.html.

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
• 06-security-auditor

How to deliver

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

Suggested delivery: gh-issue