chore: add AGENTS.md development context

AGENTS.md

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+# Repository Guidelines
+
+## Project Structure & Module Organization
+- Root entrypoint: `init.lua` (Kickstart-based primary config).
+- Custom Lua modules live under `lua/custom/`.
+- Plugin specs are split into `lua/custom/plugins/*.lua` (one concern per file, e.g. `gitsigns.lua`, `lint.lua`, `persistence.lua`).
+- Utility modules (non-plugin config) live in `lua/custom/*.lua` (for example `wrapping.lua`, `health.lua`).
+- Reference docs live in `README.md` and `doc/kickstart.txt`.
+- Plugin versions are pinned in `lazy-lock.json`.
+
+## Build, Test, and Development Commands
+- `nvim`  
+  Starts Neovim and triggers lazy.nvim plugin loading/install.
+- `nvim --headless "+qa"`  
+  Fast startup sanity check (useful in CI-style validation).
+- `nvim --headless "+checkhealth" "+qa"`  
+  Runs health checks for Neovim, plugins, and external tools.
+- `nvim --headless "+Lazy! sync" "+qa"`  
+  Syncs plugin set to current specs.
+- `luac -p init.lua lua/custom/**/*.lua`  
+  Lua syntax validation for config files.
+
+## Coding Style & Naming Conventions
+- Language: Lua (Neovim API style).
+- Formatting: `stylua` using `.stylua.toml` settings (2-space indentation, no tabs).
+- Prefer small, focused plugin spec files named by feature (`neo-tree.lua`, `markdown.lua`).
+- Use descriptive keymap `desc` fields and group prefixes via which-key.
+- Keep comments concise and practical; avoid repeating obvious code behavior.
+
+## Testing Guidelines
+- No formal unit-test framework is configured in this repo.
+- Required checks before PR:
+  - Lua parse check (`luac -p ...`)
+  - Headless startup (`nvim --headless "+qa"`)
+  - Health check (`:checkhealth`) for affected tooling (LSP, formatters, linters).
+- For plugin/config changes, include manual verification steps in PR notes (keymaps, commands, expected behavior).
+
+## Commit & Pull Request Guidelines
+- Follow existing history style: Conventional Commit-like prefixes such as:
+  - `feat(scope): ...`
+  - `fix(scope): ...`
+  - `chore: ...`
+- Keep commits scoped to one logical change (plugin, keymap group, diagnostics, etc.).
+- PRs should include:
+  - Summary of behavior changes
+  - Files touched (e.g. `init.lua`, `lua/custom/plugins/...`)
+  - Validation performed (commands run)
+  - Screenshots/GIFs only when UI behavior is materially changed.