Enable clickable file path links in message markdown

[Dimillian]

Motivation

• Messages displayed file paths as plain highlighted text but did not allow opening files or revealing them in Finder; users should be able to click a path to open it.
• Relative paths in messages should resolve against the active workspace so opener actions work from the correct project root.

Description

• Add a remark plugin remarkFileLinks and utility fileLinks to detect file-path-like text in markdown and emit codex-file: links instead of plain text.
• Extend the Markdown component to register the plugin and a custom anchor renderer that intercepts codex-file: links and calls a provided onOpenFileLink callback.
• Introduce useFileLinkOpener hook to resolve relative paths with @tauri-apps/api/path.resolve and open files using @tauri-apps/plugin-opener.openPath (ignoring opener failures to avoid breaking rendering).
• Wire the new hook into Messages by passing the active workspace path from useLayoutNodes so message-rendered links open relative to the current workspace, and add the new dependency unist-util-visit to support the remark transform.

Testing

• No automated test suite was executed for this change; no unit or E2E tests were run.
• The new dependency was installed via npm install unist-util-visit@^5.0.0 which completed successfully.

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Codex Task