chore: add code-review-flutter agent skill

[tompsota]

Summary

• Adds a project-scoped agent skill at .agents/skills/code-review-flutter/SKILL.md that codifies a strict review standard for this plugin (no hand-edits to generated files, no hardcoded fallbacks in the native layer, single source of truth across Dart/Kotlin/Swift, identifier and API alignment across all three languages, mandatory documentation of platform-specific features, and tests for new public API).
• Adds .pubignore so .agents/ (and any future .cursor/) tooling directories are excluded from pub publish.

Why

The repository didn't have a written, repo-scoped review standard. The skill captures the conventions already followed in recent reviews so any agent session opened in this repo applies them automatically. Keeping the skill in-repo (rather than personal) means it travels with the codebase and stays in lockstep with the project's structure (pigeons/, *.g.dart, android/.../generated/, ios/Classes/).

Publish-package safety

.pubignore is combined with .gitignore by pub, so adding .agents/ and .cursor/ there is sufficient to keep them out of the published archive while still letting them be tracked in git.

Verified with flutter pub publish --dry-run:

• 0 warnings
• Neither .agents/ nor .cursor/ appears in the package file listing
• All lib/, test/, pigeons/, android/, ios/, etc. content is unchanged

Test plan

• flutter pub publish --dry-run reports 0 warnings and excludes .agents/
• Open a fresh agent session in this repo and confirm the skill appears in the agent's available-skills list (verified by the maintainer once the branch is checked out locally)
• Trigger the skill with "do a code review for PR #N" and confirm it follows the workflow defined in SKILL.md

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