chore: add AGENTS.md

AGENTS.md

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+# Agent guidance
+
+This repository holds **shell scripts** for macOS/Linux: **tmux** session
+helpers under `tmux/` and **git backup** utilities under `code-backup/`. There
+is no application runtime; changes are almost always Bash and documentation.
+
+## Layout
+
+- **`tmux/`** — `setup-main.sh`, `setup-dev.sh`, `session-manager.sh`;
+  shared helpers in `tmux-utils.sh`
+- **`code-backup/`** — `code-backup-local.sh`, `code-backup-gitlab.sh`
+- **`.github/workflows/`** — PR quality checks (reusable workflow)
+
+See [README.md](README.md), [tmux/README.md](tmux/README.md), and
+[code-backup/README.md](code-backup/README.md) for usage and behavior.
+
+## Conventions (shell)
+
+- Prefer patterns already used in `tmux/`: strict mode (`set -euo pipefail`
+  where appropriate), clear errors, colored messages, logging where the rest
+  of the script logs.
+- Quote expansions; avoid word-splitting and pathname surprises on user paths.
+- `tmux-utils.sh` is **sourced** by other scripts—keep public function names
+  and side effects consistent with callers.
+- Assume **tmux** and **bash** (4.0+); do not introduce dependencies without
+  a strong reason and documentation.
+
+## Conventions (repo)
+
+- **Prettier** is configured for formatting; `package.json` only lists Prettier
+  as a dev dependency.
+- CI runs **shellcheck**, **markdownlint**, **prettier**, **yamllint**, and
+  other linters via a reusable workflow—scripts and markdown under linted
+  paths should stay clean.
+
+## What to do when editing
+
+1. Read the target script and any scripts that `source` it before changing
+   behavior.
+2. Match naming, logging, and error style of neighboring code.
+3. Run **shellcheck** on edited shell files and **prettier** on touched
+   markdown/JSON/YAML when relevant.
+4. Update the nearest `README.md` only when behavior, flags, or paths visible
+   to users change.
+
+## Out of scope
+
+- Do not add unrelated languages, frameworks, or large refactors unless the
+  user asks.
+- Do not commit secrets, tokens, or machine-specific absolute paths meant for
+  one developer only.