chore: add CONTRIBUTING.md, AGENTS.md and CLAUDE.md

AGENTS.md

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+# Agent Guidelines
+
+When you need to create branches, make commits, or open pull requests, read
+[CONTRIBUTING.md](CONTRIBUTING.md) first. No need to read it for code exploration —
+only when interacting with git.

CLAUDE.md

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+AGENTS.md

CONTRIBUTING.md

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+# Contributing
+
+## Overview
+
+This repo contains the shared AI review GitHub Actions workflow used across Jitsu repos.
+
+## Development Branch
+
+The default development branch is `main`.
+
+## Common Principles
+
+**Branch naming:** Use a type prefix — `feat/`, `fix/`, `chore/`.
+Example: `feat/add-security-checks`.
+
+**Merging policy:** We avoid merge commits. Always rebase onto the default branch —
+never merge the default branch into a branch. For PRs, merge with full history preserved
+— no squash merge. It's fine to squash overly granular commits within a branch locally
+before opening a PR.
+
+**Commit style:** [Conventional commits](https://www.conventionalcommits.org/) —
+`type(scope): description`. Common types: `fix`, `feat`, `chore`, `refactor`, `ci`.
+
+## PRs vs Direct Commits
+
+Trivial changes, bug fixes, and config updates go directly to `main`. Larger or riskier
+changes use pull requests. The engineer decides based on complexity and risk. AI review
+runs on every push and PR.