docs: add AGENTS.md describing agents and contribution workflow

Signed-off-by: Martin Wimpress <martin@wimpress.org>

AGENTS.md

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+# AGENTS.md
+
+Quickemu is a Bash wrapper around QEMU that automates VM creation for ~1000 operating systems (Linux, macOS, Windows, BSDs). Two main tools: `quickget` downloads ISOs and creates configs; `quickemu` launches VMs with optimised hardware detection.
+
+## Project structure
+
+```
+quickemu        # Main VM launcher script (Bash)
+quickget        # OS downloader and config generator (Bash)
+quickreport     # System diagnostics tool (Bash)
+chunkcheck      # Download verification utility (Bash)
+flake.nix       # Nix flake for packaging
+package.nix     # Nix package definition
+devshell.nix    # Development environment
+```
+
+## Development environment
+
+```shell
+nix develop                    # Enter devshell with all dependencies
+direnv reload                  # Update .direnv/bin/quickemu for testing
+```
+
+The devshell patches `quickemu` to use Nix store paths for OVMF and Samba, writing to `.direnv/bin/quickemu`.
+
+## Build and test commands
+
+- Lint scripts: `shellcheck quickemu quickget quickreport chunkcheck`
+- Test quickget URLs: `./quickget --check <os> [release] [edition]`
+- List all supported OSes: `./quickget --list`
+- Run VM: `./quickemu --vm <name>.conf`
+
+## Code style
+
+All scripts are Bash 4.0+ with ShellCheck compliance.
+
+| File | Indent | Notes |
+|------|--------|-------|
+| quickemu | 2 spaces | Main VM launcher |
+| quickget | 4 spaces | OS definitions and download logic |
+
+**General conventions:**
+
+- UTF-8, LF line endings, final newline
+- Trim trailing whitespace (except Markdown)
+- Functions use `function name() {` syntax
+- Many functions are invoked indirectly via dynamic dispatch - `SC2317` is disabled globally in quickget
+
+## Adding a new OS to quickget
+
+Follow the [guide in the wiki](https://github.com/quickemu-project/quickemu/wiki/06-Advanced-quickget-features#adding-a-new-os-to-quickget). Each OS requires:
+
+1. Entry in `os_info()` case statement
+2. `releases_<os>()` function returning available versions
+3. `editions_<os>()` function if multiple editions exist
+4. Download URL construction logic
+
+## Commit message format
+
+Commits must follow [Conventional Commits](https://www.conventionalcommits.org/). PR titles are validated against this format.
+
+**Common prefixes:**
+
+- `fix(quickget):` - Fix distro downloads, update editions/releases
+- `fix(quickemu):` - Fix VM configuration or runtime issues
+- `feat(quickget):` - Add new OS support
+- `feat(quickemu):` - Add new VM features
+- `ci:` - CI/workflow changes
+- `docs:` - Documentation updates
+
+**Examples from recent history:**
+
+```
+fix(quickget): update Zorin OS download method and editions
+fix(quickget): handle Solus Xfce beta naming for older releases
+fix(quickget): remove Athena OS (no longer getting updates)
+```
+
+## Pull request guidelines
+
+- PR titles must match Conventional Commits format (enforced by CI)
+- Single-commit PRs: commit message must match PR title
+- Run `shellcheck` before submitting
+- Test affected OS downloads with `./quickget --check <os> <release>`
+
+## CI workflows
+
+| Workflow | Trigger | Purpose |
+|----------|---------|---------|
+| lint-shellcheck | PR | ShellCheck with severity: warning |
+| lint-pr | PR | Validates PR title matches Conventional Commits |
+| test-quickget | PR | Tests quickget functionality |
+| test-build-quickemu | PR | Build verification |
+| flake-checker | PR | Nix flake validation |
+
+## Platform support
+
+- Host: Linux (x86_64, aarch64), macOS (x86_64, aarch64)
+- OVMF/UEFI firmware: Linux only
+- Bash 4.0+ required (explicit version check at runtime)
+
+## Key dependencies
+
+Runtime: qemu, cdrtools, curl, jq, spice-gtk, swtpm, samba, zsync
+Linux-specific: OVMF, usbutils, mesa-demos