Add StoreMembers tool and update endpoints; enhance README

.github/prompts/create-mcp-tool-from-openapi.prompt.md

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+# Reusable Prompt: Add a new MCP tool from `fsopenapi.json`
+
+Use this prompt when adding a new MCP tool to this project.
+
+---
+
+You are working in the `fsmcp` workspace.
+
+## Goal
+Add a new MCP tool for this endpoint:
+- **Endpoint path:** `<ENDPOINT_PATH>`
+- **HTTP method:** `<HTTP_METHOD>`
+
+Use `fsopenapi.json` as the source of truth for request/response schemas and constraints.
+
+## Required implementation rules
+1. **Read OpenAPI docs first**
+   - Find endpoint details in `fsopenapi.json`:
+     - summary/description
+     - parameters (path/query/body)
+     - response schema(s)
+     - relevant referenced component schemas and enums
+   - Use exact API field names via `JsonPropertyName`.
+
+2. **Update endpoint constants**
+   - Add a new endpoint constant/function in `FsMcp/Endpoints.cs`.
+   - Follow existing style (`ApiBase`, interpolation, naming consistency).
+
+3. **Create a new tool class**
+   - Create file in `FsMcp/Tools/` named after the endpoint purpose, e.g. `RegionStoresTools.cs`.
+   - Add one MCP tool method with `[McpServerTool]` and detailed `[Description]`.
+   - Inject and use `FoodsharingApiClient`.
+   - Always call `await _apiClient.EnsureLoginAsync();` before HTTP calls.
+   - Validate input parameters where appropriate (e.g. IDs > 0).
+   - Use strongly typed request/response models (`record` types).
+   - Add rich per-field `[Description]` metadata on model properties.
+   - Reuse existing shared models if already present (avoid duplication).
+
+4. **Register tool in host**
+   - Add `.WithTools<YourNewToolsClass>()` in `FsMcp/Program.cs`.
+
+5. **Extend documentation**
+   - Update `FsMcp/README.md` with a new section for the tool including:
+     - tool name
+     - endpoint + purpose
+     - input parameters
+     - auth behavior (`USERNAME`, `PASSWORD`, CSRF)
+     - output shape highlights
+     - enum/date/data-quality notes that help consumers
+
+6. **Validate**
+   - Build with:
+     - `dotnet build FsMcp/FsMcp.csproj -c Debug`
+   - If build fails due to pre-existing unrelated issues, clearly state that and confirm whether new files are clean.
+
+## Style constraints
+- Keep changes minimal and focused.
+- Match existing C# style and naming.
+- Do not add unrelated refactors.
+- Preserve API payload shape; do not rename wire-level JSON fields.
+
+## Output format expected from you
+At the end, provide:
+1. Short summary of what changed.
+2. List of changed files.
+3. Validation result.
+4. Any blockers or follow-up suggestions.
+
+## Variables to fill before running this prompt
+- `<ENDPOINT_PATH>` e.g. `/api/regions/{regionId}/stores`
+- `<HTTP_METHOD>` e.g. `GET`
+
+## Example invocation
+"Add a new MCP tool for `GET /api/regions/{regionId}/stores` using this reusable prompt and implement all required steps."