feat: add YouTrack API client and invoice generator with documentation

.factory/skills/youtrack/REFERENCES.md

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+# YouTrack API References
+
+## Core API Endpoints
+
+### Base URL
+Your YouTrack instance: `https://your-instance.youtrack.cloud/`
+API base: `https://your-instance.youtrack.cloud/api/`
+
+### Authentication
+- Use permanent token with `Authorization: Bearer <token>` header
+- To generate a token: From main navigation menu, select **Administration** > **Access Management** > **Users**, find your user, and generate a permanent API token
+
+### Projects
+
+**List all projects**
+- `GET /api/admin/projects?fields=id,name,shortName`
+- Returns array of project objects with id, name, shortName
+
+**Get specific project**
+- `GET /api/admin/projects/{projectId}?fields=id,name,shortName,description`
+
+### Issues
+
+**List issues with query**
+- `GET /api/issues?query={query}&fields={fields}`
+- Query examples:
+  - `project: MyProject`
+  - `project: MyProject updated: 2026-01-01 ..`
+  - `assignee: me`
+- Common fields to request:
+  - `id,summary,description,created,updated,project(id,name),customFields(name,value)`
+
+**Get issue**
+- `GET /api/issues/{issueId}`
+
+**Create issue**
+- `POST /api/issues`
+- Body: `{"project": {"id": "projectId"}, "summary": "Summary", "description": "Description"}`
+
+**Update issue**
+- `POST /api/issues/{issueId}`
+- Body: `{"summary": "New summary", "description": "New description"}`
+
+### Time Tracking
+
+**Get work items for an issue**
+- `GET /api/issues/{issueId}/timeTracking/workItems`
+- Returns array of work items with:
+  - `id`, `date`, `duration` (in minutes), `author`, `text`
+
+**Work item structure:**
+```json
+{
+  "id": "...",
+  "date": "2026-01-15T10:30:00.000+0000",
+  "duration": {"minutes": 30},
+  "author": {"name": "User Name", "id": "..."},
+  "text": "Work description",
+  "type": {...}
+}
+```
+
+### Knowledge Base (Articles)
+
+**List articles**
+- `GET /api/articles?project={projectId}`
+
+**Get article**
+- `GET /api/articles/{articleId}`
+
+**Create article**
+- `POST /api/articles`
+- Body: `{"project": {"id": "projectId"}, "title": "Title", "content": "Content"}`
+
+## Query Language Examples
+
+```
+project: MyProject
+project: MyProject assignee: me
+project: MyProject updated >= 2026-01-01
+priority: Critical
+has: time
+```
+
+**Note:** Date filtering in REST API uses `updated >= YYYY-MM-DD` format, not `updated: date ..` format.
+
+## Field IDs Reference
+
+Common custom field IDs (may vary by setup):
+- Priority: `priority`
+- State: `State`
+- Assignee: `Assignee`
+- Due Date: `Due Date`
+- Type: `Type`
+
+Check your instance's actual field IDs in YouTrack UI or via API.