Fallout.Extensions.MCP-Server: ship an MCP server as a standalone product

### Problem

The only way for an AI agent to drive Fallout today is to run the CLI and parse console text. MCP (Model Context Protocol, the standard interface agents use to reach external tools) would let an agent call typed operations instead. Nothing in this repository references MCP.

#240 (project-local AI tooling config) covers *consuming* MCP servers inside this repository, which is the opposite direction from shipping one.

### Outcome

A standalone product, `Fallout.Extensions.MCP-Server`, in its own repository alongside the other extension products such as `Fallout.Extensions.VSCode`. It exposes Fallout's build model and run results as MCP tools.

Proposed tools, each a thin layer over CLI work tracked separately in this milestone:

- `describe` — the build model as data
- `plan` — what a target would run, without running it
- `run` — execute a target, with the effect grant passed explicitly
- `last_run` — read the durable run summary

### Acceptance criteria

- [ ] New repository `Fallout-build/Fallout.Extensions.MCP-Server`, C#, built on the official `ModelContextProtocol` NuGet package
- [ ] Server locates and loads a Fallout build project in a given directory
- [ ] `describe`, `plan`, `run` and `last_run` implemented, with `run` gated on an explicit effect grant
- [ ] Shipped as something a user can install and register with an MCP client
- [ ] Versioned independently of the Fallout host

### Notes

Build order matters: the `describe --json` command and the durable run summary are the data layer this sits on. Building the server first would mean screen-scraping console output, which is the problem this milestone exists to remove.

The official C# SDK is [`ModelContextProtocol`](https://github.com/modelcontextprotocol/csharp-sdk), maintained in collaboration with Microsoft, so this product stays on the C# stack. The VSCode extension is TypeScript out of necessity; this one is not.