Adding agent plugin and documentatoin (#333)

Closes #269 

## Description

ships a Copilot plugin for the repo itself.


### Copilot Plugin (`.github/plugin/`)

Ships a repo-level Copilot plugin with an agent file and the same 7
skills:

```
.github/plugin/
├── plugin.json
├── agents/
│   └── winapp-cli/
│       └── winapp.agent.md      # system prompt
└── skills/
    └── winapp-cli/
        ├── setup/SKILL.md
        ├── package/SKILL.md
        ├── identity/SKILL.md
        ├── signing/SKILL.md
        ├── manifest/SKILL.md
        ├── troubleshoot/SKILL.md
        └── frameworks/SKILL.md
```

The agent file (`winapp.agent.md`) includes:
- Full command reference with syntax + descriptions
- Decision tree for common workflows
- Framework-specific packaging guidance
- Error diagnosis patterns
- Critical rules (e.g. "Run `winapp --cli-schema` for the full CLI
reference")

### Simplified AI Documentation

Removed redundant docs in favor of the skill-based approach:
- **Deleted** `docs/llm-context.md` (full CLI reference doc — now
covered by skills + `winapp --cli-schema`)
- **Deleted** `docs/using-with-llms.md` (guide — now covered by README
section + skills)
- **Merged** two README AI sections into a single concise "🤖 Using with
AI Coding Agents" section
- **Updated** `llms.txt` with Copilot plugin structure

## Type of Change

<!-- Keep the applicable line(s), delete the rest -->

- ✨ New feature
- 💥 Breaking change
- 📝 Documentation
- 🔧 Config/build
- 🧪 Test update

## Checklist
<!-- Delete the ones that do not apply to your changes -->

- [x] New tests added for new functionality (if applicable)
- [x] Tested locally on Windows
- [x] Main [README.md](../README.md) updated (if applicable)
- [x] [docs/usage.md](../docs/usage.md) updated (if CLI commands
changed)

---------

Co-authored-by: Nikola Metulev <711864+nmetulev@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Alexandre Zollinger Chohfi <alzollin@microsoft.com>

Author: 3 people

.github/PULL_REQUEST_TEMPLATE.md

@@ -36,6 +36,7 @@ winapp store app list
 - [ ] [docs/usage.md](../docs/usage.md) updated (if CLI commands changed)
 - [ ] [Language-specific guides](../docs/guides) updated (if applicable)
 - [ ] [Sample projects updated](../samples) to reflect changes (if applicable)
+- [ ] Agent skill templates updated in `docs/fragments/skills/` (if CLI commands/workflows changed)
 
 ## Screenshots / Demo