[ACTION] Figma: Add list/search files action to fetch File IDs

**Is there a specific app this action is for?**
Figma

**Please provide a link to the relevant API docs for the specific service / operation.**
https://www.figma.com/developers/api

## Problem

The `fileId` prop in the Figma app is a free-form text field with no async options. An agent cannot discover available file IDs — it must know the exact key in advance, which requires a human to extract it from a Figma URL. This blocks all downstream actions that depend on `fileId`, most critically the comment actions: `commentId` has async options but they depend on `fileId` being resolved first. If `fileId` is unknown, comment-related actions are entirely unusable by an agent.

## What's needed

Add a **Find Files** action (or `list-files` / `search-files` action) that accepts a `projectId` and returns a list of files with their keys and names. This lets an agent resolve a file name to its ID before calling comment or other file-scoped tools.

The `fileId` propDefinition should also be updated to include async options that fetch files from a given project via the Figma API.

## Notes

- The Figma API supports listing files within a project: `GET /v1/projects/{project_id}/files`
- Resolving `fileId` depends on having a `projectId` first — PIPE-17 must also be addressed, as `projectId` is equally free-form
- Once `fileId` is resolvable, `commentId` already has async options and will work correctly

***[PIPE-18]***