feat: agent plugin with SKILL.md-based instruction generation (#75)

* feat: agent plugin with SKILL.md-based instruction generation

Replace hardcoded inline prompts with SKILL.md assets loaded via the
Copilot SDK skillDirectories option and invoked with /skill-name prefixes.

- Add plugin/ directory with 4 skills: root-instructions, area-instructions,
  nested-hub, nested-detail
- Add plugin manifest (plugin/.github/plugin/plugin.json) and marketplace
  manifest (.github/plugin/marketplace.json)
- Add skills.ts resolver with dev/bundled/extension path resolution
- Update instructions.ts to use skillDirectories + /skill-name invocation
- Copy skill assets at build time (tsup onSuccess, esbuild onEnd)
- Initialize skills dir in VS Code extension activate()
- Add tests with filesystem assertions and SKILL.md frontmatter validation
- Update docs: copilot-instructions.md, CHANGELOG, README, plugin/README.md

* fix: address PR review feedback

- CRLF-safe frontmatter parser in skills test (fixes Windows CI)
- Clean dist/skills/ and out/skills/ before copy to remove stale assets
- Fix AGENTS.md typo and add .instructions.md to root-instructions glob
- Add explicit marketplace manifest URL in plugin/README.md
- Migrate .github/copilot-instructions.md → AGENTS.md

---------

Co-authored-by: Harald Kirschner <digitarald@gmail.com>

Author: digitarald

.github/copilot-instructions.md

@@ -0,0 +1,24 @@
+{
+  "name": "agentrc-plugins",
+  "metadata": {
+    "description": "AgentRC plugins for GitHub Copilot — skills for AI-assisted repository priming",
+    "version": "1.0.0"
+  },
+  "owner": {
+    "name": "Microsoft"
+  },
+  "plugins": [
+    {
+      "name": "agentrc",
+      "source": "./plugin",
+      "description": "Skills for generating instruction files (copilot-instructions.md, area instructions, AGENTS.md) by analyzing repository structure and conventions.",
+      "version": "2.1.0",
+      "skills": [
+        "./skills/root-instructions",
+        "./skills/area-instructions",
+        "./skills/nested-hub",
+        "./skills/nested-detail"
+      ]
+    }
+  ]
+}

.github/plugin/marketplace.json

@@ -0,0 +1,67 @@
+# AgentRC Copilot Instructions
+
+`@microsoft/agentrc` is a CLI tool (and VS Code extension) that sets up repositories for AI-assisted development. It analyzes repos, generates instruction files, evaluates AI responses, and runs readiness reports.
+
+## Architecture
+
+```
+src/                   # CLI entry point and commands
+  cli.ts               # Commander program, withGlobalOpts helper
+  commands/            # Thin command wrappers (parse args → call services)
+  ui/                  # Ink/React TUI components
+packages/core/src/     # Shared services and utilities (bundled into dist)
+  services/            # analyzer, generator, instructions, readiness, policy, ...
+  utils/               # fs, output, logger, repo, pr
+plugin/skills/         # Built-in skill markdown files (copied to dist/skills/ on build)
+vscode-extension/      # See .github/instructions/vscode-extension.instructions.md
+```
+
+`@agentrc/core` (in `packages/core/`) is **bundled** into the CLI output — it is not an external npm package. Use the `@agentrc/core/*` path alias for all cross-package imports.
+
+## Build & Test
+
+```sh
+npm run build          # tsup → dist/index.js (ESM, Node 20)
+npm run dev            # tsx src/index.ts (run from source)
+npm run typecheck      # tsc --noEmit
+npm run lint           # eslint
+npm test               # vitest run
+npm run test:coverage  # vitest run --coverage
+```
+
+Output is ESM (`"type": "module"`). All node_modules are external except `@agentrc/core`, `@github/copilot-sdk`, and `vscode-jsonrpc` (bundled via `noExternal` in `tsup.config.ts`).
+
+> **SDK shim**: `tsup.config.ts` patches `getBundledCliPath()` in the Copilot SDK. An identical shim lives in `vscode-extension/esbuild.mjs` — update **both** together if SDK internals change.
+
+## Command Pattern
+
+Commands in `src/commands/` are thin wrappers:
+
+1. Resolve paths and options
+2. Call services from `@agentrc/core/services/*`
+3. Write output using the output utilities
+
+Wrap every command action with `withGlobalOpts` (from `src/cli.ts`) so `--json`, `--quiet`, and `--accessible` are merged into the options object.
+
+## Output Convention
+
+| Stream   | Purpose                                           |
+| -------- | ------------------------------------------------- |
+| `stdout` | Machine-readable JSON only (when `--json` is set) |
+| `stderr` | Human-readable progress, logs, warnings, errors   |
+
+Never write progress to stdout. Use `outputResult` / `outputError` from `@agentrc/core/utils/output`. Use `shouldLog(options)` before writing stderr text. Use `createProgressReporter(silent)` for step-by-step progress.
+
+`CommandResult<T>` shape: `{ ok, status: "success"|"partial"|"noop"|"error", data?, errors? }`.
+
+## Skills
+
+Built-in skills live in `plugin/skills/<skill-name>/SKILL.md` with YAML frontmatter (`name`, `description`). They are copied to `dist/skills/` on build via `tsup.config.ts`'s `onSuccess` hook. Resolve the skills directory with `getSkillDirectory()` from `@agentrc/core/services/skills` — never hardcode paths.
+
+## Code Conventions
+
+- **Type imports**: always use `import type` for type-only imports (`@typescript-eslint/consistent-type-imports` is enforced).
+- **Import order**: alphabetical, grouped by node built-ins → external packages → internal (`@agentrc/core/*`) → relative. ESLint enforces this.
+- **Tests**: test files live in `src/services/__tests__/` and use vitest. Default timeout is 10 s.
+- **Unused vars**: prefix with `_` to suppress the lint warning.
+- **Strict TypeScript**: `strict: true`, `moduleResolution: Bundler`, target `ES2022`.

AGENTS.md

@@ -6,6 +6,8 @@ All notable changes to this project will be documented in this file.
 
 ### Features
 
+- **Agent plugin & marketplace** — expose built-in instruction-generation skills as a VS Code agent plugin (`plugin/`) with marketplace manifest (`.github/plugin/marketplace.json`). Skills can be installed from source or discovered through the marketplace.
+- **SKILL.md-based prompts** — replace hardcoded inline prompts with SKILL.md assets loaded via the Copilot SDK `skillDirectories` option and invoked with `/skill-name` prefixes.
 - **Enhanced .NET detection** — F# support, framework parsing, and expanded signals (#60)
 - **De-branded terminology** — `--dry-run` flag, batch instructions command (#55)
 

CHANGELOG.md

@@ -83,6 +83,7 @@ Works with **GitHub** and **Azure DevOps**. Supports monorepos, multi-root VS Co
 | **[At Scale](docs/at-scale.md)**               | Batch processing across orgs                            |
 | **[CI Integration](docs/ci-integration.md)**   | GitHub Actions & Azure Pipelines                        |
 | **[VS Code Extension](docs/extension.md)**     | Sidebar views, commands, settings                       |
+| **[Agent Plugin](plugin/README.md)**           | Install as a Copilot agent plugin with built-in skills  |
 | **[Examples](examples/)**                      | Configs, evals, and policies                            |
 
 [Customize AI in VS Code](https://code.visualstudio.com/docs/copilot/customization/overview) · [Custom instructions](https://code.visualstudio.com/docs/copilot/customization/custom-instructions) · [CONTRIBUTING.md](CONTRIBUTING.md)

README.md

@@ -90,6 +90,9 @@ export {
   stripMarkdownFences
 } from "./services/instructions";
 
+export { setBuiltinSkillsDir, getSkillDirectory } from "./services/skills";
+export type { BuiltinSkillName } from "./services/skills";
+
 export { parsePolicySources, loadPolicy, resolveChain } from "./services/policy";
 
 export {