feat(examples): allow disabling the pi extension via GONDOLIN_DISABLED

[ColonelPanics]

Summary

• Adds a lightweight escape hatch to the pi-gondolin.ts example extension: when GONDOLIN_DISABLED=1 is set, the extension default-export returns early, so pi falls back to its normal host tools while the extension itself stays loaded.
• Documents the flag in the file header notes.

Motivation

Since the example overrides pi's built-in read/write/edit/bash tools, the only way to run pi without the sandbox while keeping other extensions, skills, and MCP servers intact is to launch with --no-extensions (-ne) — which disables everything else too, or to edit settings to remove the extension.

A common workflow is:

alias pi-host='GONDOLIN_DISABLED=1 pi'

pi runs sandboxed in the VM as usual; pi-host runs with real host access and all other extensions enabled. Because the VM is started lazily and the early return prevents all tool re-registration and the session_start/user_bash hooks from doing anything, this adds no overhead or behavior change when unset.

The env var name follows the existing GONDOLIN_* convention (GONDOLIN_CPU, GONDOLIN_KRUN_RUNNER, GONDOLIN_START_TIMEOUT_MS, …).

Test plan

• Verified locally against a personal install of the extension: pi runs as before (VM routing active), GONDOLIN_DISABLED=1 pi uses host tools with the extension loaded but inert.

Note: open PR #125 restructures where the pi extension lives (host/extensions/). If that lands first I'm happy to rebase this onto it.