feat: port CLI, TUI, and launcher from v1.5 into v2 (#1452)

Ports the CLI, TUI, and launcher from v1.5 into v2 behind a single
`mcp-inspector` entrypoint (launcher → web/cli/tui). Includes the
`mcp-inspector --web --dev` launcher fix (Vite alias resolution + prod-server
double-start), the root `postinstall` install cascade with simplified CI, and
spec/AGENTS docs (no-workspaces rationale, Development setup).

Closes #1246.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/install-clients.mjs

@@ -0,0 +1,65 @@
+#!/usr/bin/env node
+/**
+ * Root postinstall: install dependencies for each client package.
+ *
+ * v2 deliberately does not use npm workspaces (see the rationale in
+ * specification/v2_cli_tui_launcher.md), so each client under clients/* keeps
+ * its own package.json and node_modules. Without this cascade a from-source
+ * checkout would have to run `npm install` in every client directory by hand —
+ * the install-friction gap. Running it from the root `postinstall` makes a
+ * single `npm install` at the repo root populate every client.
+ *
+ * Safe outside a source checkout:
+ *  - When this package is installed as a dependency it lives under a
+ *    node_modules directory; we detect that and exit early.
+ *  - The published tarball ships only each client's build/ output (see the
+ *    root package.json `files` array), not their package.json, so even if the
+ *    node_modules check were bypassed the per-client guard finds nothing to
+ *    install and the script no-ops.
+ *
+ * Set INSPECTOR_SKIP_CLIENT_INSTALL=1 to skip explicitly — e.g. CI that
+ * installs each client itself and wants to avoid the redundant pass.
+ */
+
+import { spawnSync } from "node:child_process";
+import { existsSync } from "node:fs";
+import { dirname, join, resolve, sep } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const repoRoot = resolve(dirname(fileURLToPath(import.meta.url)), "..");
+const CLIENTS = ["web", "cli", "tui", "launcher"];
+
+if (process.env.INSPECTOR_SKIP_CLIENT_INSTALL) {
+  console.log(
+    "[install-clients] INSPECTOR_SKIP_CLIENT_INSTALL set — skipping client installs.",
+  );
+  process.exit(0);
+}
+
+// Installed as a dependency (lives under node_modules) → nothing to do.
+if (repoRoot.split(sep).includes("node_modules")) {
+  process.exit(0);
+}
+
+for (const name of CLIENTS) {
+  const dir = join(repoRoot, "clients", name);
+  // The published tarball ships build/ output only, no client package.json.
+  if (!existsSync(join(dir, "package.json"))) {
+    continue;
+  }
+  console.log(
+    `[install-clients] Installing dependencies for clients/${name}...`,
+  );
+  const result = spawnSync("npm", ["install"], {
+    cwd: dir,
+    stdio: "inherit",
+    // npm is npm.cmd on Windows, which needs a shell to resolve.
+    shell: process.platform === "win32",
+  });
+  if (result.status !== 0) {
+    console.error(
+      `[install-clients] npm install failed in clients/${name} (exit ${result.status ?? "unknown"}).`,
+    );
+    process.exit(result.status ?? 1);
+  }
+}