fix(config pi): embed extension sources so installed binary is self-contained

- add scripts/generate-pi-extension.ts to embed src/pi-ext sources as
  string constants (mirrors prime-agent's embedded switcher pattern).
- config pi now writes the embedded sources by default, so it works from
  an /usr/bin install with no source tree (--extension-dir overrides for
  dev). Verified installed binary writes files byte-identical to source.
- add installEmbeddedExtension test.

scripts/generate-pi-extension.ts

@@ -0,0 +1,48 @@
+#!/usr/bin/env node
+/**
+ * Generates `src/cli/src/config/pi-extension.ts` which embeds the pi extension
+ * sources (`mcpctl-pi.ts` + `mcp-http.ts`) as string constants, mirroring how
+ * prime-agent's `/mcpctl` switcher is embedded (`prime-agent-extension.ts`).
+ *
+ * Embedding matters: `mcpctl config pi` must work from an installed binary
+ * that has no access to the source tree. The installed extension files are this
+ * exact embedded source, so what the CLI ships is always what pi runs.
+ *
+ * Regenerate after editing the extension sources:
+ *   npx tsx scripts/generate-pi-extension.ts
+ */
+import { readFileSync, writeFileSync, mkdirSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+
+const scriptsDir = import.meta.dirname;
+const root = join(scriptsDir, '..');
+const piExtDir = join(root, 'src', 'pi-ext');
+
+const MAIN = readFileSync(join(piExtDir, 'mcpctl-pi.ts'), 'utf-8');
+const HTTP = readFileSync(join(piExtDir, 'mcp-http.ts'), 'utf-8');
+
+function embed(src) {
+  // JSON.stringify yields a quoted string literal we can inline directly.
+  return JSON.stringify(src);
+}
+
+const out = `/**
+ * Embedded source of the mcpctl pi extension — DO NOT EDIT BY HAND.
+ * Generated by \`npx tsx scripts/generate-pi-extension.ts\` from
+ * \`src/pi-ext/mcpctl-pi.ts\` and \`src/pi-ext/mcp-http.ts\`.
+ *
+ * \`mcpctl config pi\` writes these verbatim into ~/.pi/agent/extensions/mcpctl/
+ * (and registers them), so an installed binary with no source tree can still
+ * provision a working pi integration.
+ */
+export const PI_EXTENSION_FILENAMES = ['mcpctl-pi.ts', 'mcp-http.ts'] as const;
+
+export const PI_EXTENSION_FILES: Record<string, string> = {
+  'mcpctl-pi.ts': ${embed(MAIN)},
+  'mcp-http.ts': ${embed(HTTP)},
+};
+`;
+
+mkdirSync(dirname(join(root, 'src', 'cli', 'src', 'config')), { recursive: true });
+writeFileSync(join(root, 'src', 'cli', 'src', 'config', 'pi-extension.ts'), out);
+console.log('wrote src/cli/src/config/pi-extension.ts');