chore(dev): Add dev runner and split dev ports

Add dev:web (rsbuild) and dev:cli (tttk serve ./content), with dev
running both via scripts/dev.mjs, which shuts both down when either
exits. Also add tttk:inspect for --inspect debugging.

Both servers previously defaulted to port 3000, so the CLI could not
bind while the web dev server was running and the proxy targeted
itself. Pin the web dev server to 5173; the CLI keeps 3000 to match
the proxy. Document the two-process setup and inspector debugging in
docs/development.md.

scripts/dev.mjs

@@ -0,0 +1,55 @@
+#!/usr/bin/env node
+/**
+ * Dev runner: starts the CLI content server (port 3000) and the rsbuild web
+ * dev server (port 5173) together, then tears both down when either exits.
+ *
+ *   npm run dev       # both (this script)
+ *   npm run dev:cli   # CLI content server only
+ *   npm run dev:web   # web dev server only
+ */
+import { spawn } from "node:child_process";
+
+const isWindows = process.platform === "win32";
+const children = new Set();
+let shuttingDown = false;
+
+/** Terminate a child and everything it spawned. */
+function killTree(child) {
+  if (child.exitCode !== null || child.signalCode !== null) return;
+  if (isWindows) {
+    spawn("taskkill", ["/pid", String(child.pid), "/T", "/F"], {
+      stdio: "ignore",
+    });
+  } else {
+    try {
+      // Negative pid targets the process group created via `detached`.
+      process.kill(-child.pid, "SIGTERM");
+    } catch {
+      child.kill("SIGTERM");
+    }
+  }
+}
+
+function shutdown(code) {
+  if (shuttingDown) return;
+  shuttingDown = true;
+  for (const child of children) killTree(child);
+  process.exitCode = code ?? 0;
+}
+
+for (const script of ["dev:cli", "dev:web"]) {
+  const child = spawn("npm", ["run", script], {
+    stdio: "inherit",
+    shell: true,
+    detached: !isWindows,
+  });
+  children.add(child);
+  child.on("exit", (code) => shutdown(code ?? 0));
+  child.on("error", (error) => {
+    console.error(`[dev] Failed to start ${script}: ${error.message}`);
+    shutdown(1);
+  });
+}
+
+process.on("SIGINT", () => shutdown(0));
+process.on("SIGTERM", () => shutdown(0));