Add first-run setup wizard + doctor diagnostics

New users hit predictable friction (no npm install, Remote Control / Python
plugin not enabled, C++ not built). This makes onboarding self-service:

- src/doctor.js — shared diagnostics (project & engine detection, Node deps,
  Remote Control reachability, Python plugin, C++ module built, optional Voxel).
  Zero-dependency top-level imports so it runs before `npm install`. Runnable as
  `npm run doctor`, reused by the wizard and the server first-run hook.
- src/setup.js — interactive `npm run setup` wizard: offers to install deps,
  confirms/pins project & engine (and RC URL) overrides, re-checks the editor
  until reachable, then prints the MCP-client config snippet.
- projectPaths.js — persisted config (mcp.config.json, git-ignored) with
  precedence env > config > auto-detect; remoteControlUrl(); first-run marker
  helpers (isFirstRun/markRun under Saved/).
- server.js — `doctor` MCP tool (so a client/assistant can run the checks in a
  session) + a one-time first-run diagnostic printed to stderr pointing at
  `npm run setup`.
- ueBridge.js honors the config-driven Remote Control URL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -40,13 +40,18 @@ It has two halves that ship together:
 4. In the editor, enable **Remote Control** and start its web server
    (`WebControl.StartServer`, or set `WebControl.EnableServerOnStartup=true`).
 
-5. Install the MCP server deps:
+5. **Run the setup wizard** — it installs deps, confirms the auto-detected
+   project/engine, checks the editor side, and prints the exact client-config
+   snippet to paste:
 
    ```bash
    cd Plugins/UEBlueprintMCP
-   npm install
+   npm run setup
    ```
 
+   (Prefer to do it by hand? `npm install`, then point your client at
+   `src/server.js` as below.)
+
 6. Point your MCP client at `src/server.js`, e.g.:
 
    ```json
@@ -62,6 +67,20 @@ It has two halves that ship together:
 
    Call `ping_ue` first to confirm the editor is reachable.
 
+### First-run check & `doctor`
+
+The first time the server starts it prints a one-time environment check to its
+stderr (visible in your client's MCP logs) and points you at `npm run setup` if
+anything is off. You can re-run the checks any time:
+
+- `npm run doctor` — print the diagnostics report in a terminal.
+- the **`doctor`** MCP tool — same checks from inside a session, so the assistant
+  can diagnose and guide you.
+
+The wizard pins any overrides (project dir, engine dir, Remote Control URL, ...)
+to a git-ignored `mcp.config.json`. Precedence everywhere is **env var > config >
+auto-detect**.
+
 ---
 
 ## Zero-config path resolution