feat: author check-downstream-consumers skill and known consumers resource

[reidbaker-agent]

Description

Authors the check-downstream-consumers contributor skill (tool/dart_skills_lint/.agents/skills/check-downstream-consumers/SKILL.md) and bundled reference resource (resources/known_consumers.md).

Objectives & Capabilities

• Systemic PR Impact Evaluation: Structures the end-to-end process for validating active pull requests and feature branches against downstream ecosystem repositories (flutter/flutter, flutter/devtools, dart-lang/site-www, etc.).
• Zero-Commit Consumer Guardrail: Enforces strict boundaries prohibiting the agent from staging, committing, or pushing code across external downstream repositories (pubspec.yaml mutates and calling updates remain strictly diagnostic/transient and must be cleanly restored before concluding).
• Dynamic Checkout Discovery: Directs autonomous agents to leverage existing workspace contexts, machine-specific Knowledge Items (KIs), or localized parent inspections rather than triggering unbounded disk traversals (find / -name ...).
• Human-in-the-Loop Decision Gates: Structures explicit pause points when breaking API usage occurs so reviewers can consciously guide the agent between Pathway A (mitigating breakages with backwards-compatible shims/tests) and Pathway B (verifying downstream migrations against CHANGELOG.md).

Quality & Verification

• Updated tool/dart_skills_lint/.agents/skills/.gitignore (/*) to cleanly index un-ignored contributor skills without wildcard recursion traps.
• Evaluated cleanly against dart format ., dart analyze --fatal-infos, and the internal skills validator test suite (test/dart_skills_lint_skills_test.dart).

[gemini-code-assist]

Code Review

This pull request introduces a new 'check-downstream-consumers' skill to the 'dart_skills_lint' agent, providing documentation and a structured process for evaluating breaking changes against known downstream repositories. The changes include updates to the .gitignore file, a detailed SKILL.md guide, and a list of known consumer repositories. The reviewer provided actionable feedback to correct a repository URL in the documentation and suggested using numbered lists for sequential steps to improve readability.