Support dragging Finder files and folders into the composer as path mentions

[hbw00111]

Problem

The composer already accepts path mentions dragged from T3 Code's internal file tree through #4140. Dragging from Finder or another OS file manager takes a different path, so regular files and folders outside the current tree cannot be referenced with the same workflow.

This is especially noticeable for folders, files outside the repository, and filenames containing spaces: users have to switch context, copy the path, return to T3 Code, and format the reference manually.

Proposed behavior

On desktop, when the app host and the selected environment are the same local machine and OS, dropping regular files or folders from the system file manager onto the composer should insert the same structured path mentions used by internal file-tree drag and drop.

Images should continue through the existing image-attachment flow. Web behavior should remain unchanged. For SSH, WSL, containers, or any other environment where a host path is not valid in the target, the composer should not turn the local path into a mention or send that path to the remote agent.

The useful first version would cover single and multiple drops, files and directories, paths containing spaces, and a visible drop target consistent with the current composer.

Why this boundary

It keeps the feature predictable: a generated mention always points to something the active agent can actually access. It also extends the interaction introduced in #4140 without changing attachment semantics or leaking meaningless local paths into remote sessions.

Prepared with GPT-5.6 Sol through the T3 Code Codex harness.