Add agent guidance and update contributor workflow docs

[WellDunDun]

Summary

• Add a top-level AGENTS.md with Frontier-specific workflow, invariants, test selection, and safety guidance for coding agents.
• Introduce repo-local development skill docs for implement, fix-tests, and code-review workflows under .agents/skills/.
• Update contributor and PR templates to point at the new guidance and validation expectations.
• Clarify in README.md that agent-facing maintenance skills are separate from shipped plugin skills.

Testing

• Not run (not requested).

Summary by CodeRabbit

• Documentation

  • Added comprehensive development guidance covering architecture invariants, testing strategies, and security practices
  • Clarified separation between local development workflows and shipped plugin features
  • Updated contribution workflow with targeted testing recommendations

• Chores

  • Added new workflow skill documents for code review, test diagnostics, and change implementation
  • Enhanced PR template validation procedures

[coderabbitai]

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Free

Run ID: 1f86c11b-4632-4a58-9acf-e5e402d5fc44

📥 Commits

Reviewing files that changed from the base of the PR and between 1906bd3 and 044fd0f.

📒 Files selected for processing (7)

• .agents/skills/frontier-code-review/SKILL.md
• .agents/skills/frontier-fix-tests/SKILL.md
• .agents/skills/frontier-implement-change/SKILL.md
• .github/PULL_REQUEST_TEMPLATE.md
• AGENTS.md
• CONTRIBUTING.md
• README.md

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📝 Walkthrough

Walkthrough

This PR establishes comprehensive guidance for AI agents and developers working on the Frontier repository. It introduces a new AGENTS.md reference document with architecture invariants and test matrices, three reusable skill workflows for code review, test fixing, and feature implementation, and integrates this guidance into existing developer documentation and PR templates.

Changes

Agent Guidance and Developer Integration

Layer / File(s)	Summary
Core agent repository guidelines AGENTS.md	AGENTS.md provides comprehensive guidance covering project overview, command reference, development workflow, test selection matrix, architecture invariants (fail-closed on missing state, no implicit fallbacks), security/data handling rules, and documentation standards for AI agents working in the Frontier repository.
Agent workflow skill definitions .agents/skills/frontier-code-review/SKILL.md, .agents/skills/frontier-fix-tests/SKILL.md, .agents/skills/frontier-implement-change/SKILL.md	Three skill documents define ordered agent workflows: frontier-code-review (scope determination, full file review, test/AGENTS.md checks, prioritized blocking issues), frontier-fix-tests (failure reproduction, inspection, classification, preferred implementation fixes, regression coverage), and frontier-implement-change (identify boundaries, add isolated tests, implement in plugin scripts, run checks, validate CLI).
Developer workflow and PR template updates CONTRIBUTING.md, .github/PULL_REQUEST_TEMPLATE.md, README.md	CONTRIBUTING.md, PR template, and README are updated to reference AGENTS.md for agent-facing guidance, specify when to run targeted tests and plugin validation, clarify that .agents/skills/ contains repo-local maintenance workflows (separate from shipped plugins/frontier/skills/), and establish development best practices aligned with agent guardrails.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 A warren of wisdom, now written so clear,
For agents and developers to hold most dear!
Guard the invariants, test what you build,
Let AGENTS.md's map keep the failures fulfilled.
Three skills light the way through code, fix, and review—
A bouncy new workflow for all that we do! 🌟

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Note

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