Add GitHub Copilot agent instructions (#12)

Author: csutter

.github/copilot-instructions.md

@@ -0,0 +1,62 @@
+# Copilot Instructions
+
+## Structure
+
+Each feature lives under `src/<feature-name>/` and has a parallel test directory at `test/<feature-name>/`:
+
+```
+src/<feature-name>/
+  devcontainer-feature.json   # Metadata, mounts, env vars, postCreateCommand
+  install.sh                  # Runs at image build time (root, inside container)
+  NOTES.md                    # Human-written design notes — update when making meaningful changes
+  README.md                   # Auto-generated — DO NOT edit directly, run `mise run docs`
+test/<feature-name>/
+  test.sh                     # Integration test run inside container after feature install
+```
+
+## Adding a Feature
+
+1. Create `src/<feature-name>/devcontainer-feature.json`, `install.sh`, and `NOTES.md`
+2. Add `test/<feature-name>/test.sh` covering every installed binary and configured file
+3. Add the new feature to the top-level `README.md` features list
+4. Run `mise run docs` to generate `src/<feature-name>/README.md`
+
+## `install.sh` Conventions
+
+- Shebang: `#!/usr/bin/bash`; `set -e` at the top
+- Detect distro by sourcing `/etc/os-release` and switching on `$ID`
+- Support Debian, Ubuntu, and Fedora at minimum; `exit 1` on unknown distros
+- Watch for package name differences across distros (e.g. `openssh-client` vs `openssh-clients`)
+
+See [src/personal-setup/install.sh](../src/personal-setup/install.sh) for the reference pattern.
+
+## Tests
+
+Tests use `dev-container-features-test-lib` (injected by the devcontainer test runner):
+
+```bash
+source dev-container-features-test-lib
+check "label"  bash -c "..."
+reportResults
+```
+
+- Add a `check` for every binary installed and every file/env var configured
+- Always write tests for new functionality before considering it complete
+- Run tests: `mise run test` (tests against debian, ubuntu, fedora-toolbox, devcontainers/base:ubuntu)
+
+## Before Committing
+
+```sh
+mise run lint-shell   # ShellCheck all .sh files
+mise run docs         # Regenerate README.md from devcontainer-feature.json + NOTES.md
+mise run test         # Full integration test suite (requires Docker)
+```
+
+## NOTES.md
+
+Update `src/<feature-name>/NOTES.md` when:
+- Adding or removing a package and there's a non-obvious reason why
+- Changing the dotfiles/mount/RCM strategy
+- Documenting a tradeoff or design decision for future reference
+
+`README.md` is auto-generated and will include the contents of `NOTES.md` — so write `NOTES.md` as if it's public-facing prose.