Install and manage custom Pi extensions from the UI

[CommanderTvis]

Problem

Users who want to use custom Pi extensions (e.g. npm:pi-file-injector) currently have no way to discover, install, or manage them in ThinkRail.

Because ThinkRail runs Pi embedded in-process, users often will not have the standalone pi CLI binary installed on their PATH. For such users, all standard instructions or documentation recommending pi install <source> fail with command-not-found unless the user knows how to manually edit ~/.pi/agent/settings.json, drop files into ~/.pi/agent/extensions/, or run npx @earendil-works/pi-coding-agent install ....

Proposed solution

Provide a way in ThinkRail to install and manage Pi extensions directly from the UI:

• An Extensions section in Settings to install extensions by its spec (e.g. npm:<pkg>, git:<url>).
• Under the hood, leverage Pi's package manager (DefaultPackageManager) to resolve and install packages into ~/.pi/agent/npm or project .pi/ and persist them to settings.json.
• List currently installed extensions, their source (bundled vs. user/project), and allow toggling or removing them.

Alternatives considered

• Requiring users to install the pi CLI globally via npm install -g @earendil-works/pi-coding-agent or run commands via npx / bunx.
• Requiring users to manually edit ~/.pi/agent/settings.json or .pi/settings.json.
• Having users manually copy extension scripts into ~/.pi/agent/extensions/.

Additional context

Related to #203 (pi extension management).

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