feat(desktop): make Markdown file references in sessions openable

Problem

When a session mentions or links to a Markdown document in the local workspace, there is no convenient way to open that document from the conversation.

Users must manually copy the path, switch to Finder or another editor, navigate to the file, and open it. This interrupts the workflow, especially when an agent creates, edits, or cites several .md documents during a task.

The existing Files tab in the right session workbar can preview generated files, but file references appearing in the transcript are not consistently actionable, particularly for existing workspace documents rather than newly generated artifacts.

Desired outcome

Local Markdown file references in a session should be interactive. When the user selects a referenced .md file, Maka should offer one or both of these actions:

1. Preview in Maka — open the document in the right-side Files/workbar preview without replacing the conversation.
2. Open locally — open the file with the system's default application, or reveal it in Finder/Explorer so the user can choose how to view or edit it.

Suggested interaction and acceptance scenarios:

• Markdown links and recognizable workspace-relative file references in assistant/tool output can be clicked.
• Selecting Preview opens the Markdown document in the existing right-side workbar and renders it readably.
• Selecting Open locally opens the file with the default application, while Reveal in Finder/Explorer selects the file in its containing folder.
• Relative paths resolve against the session's project/workspace root.
• Paths containing spaces or non-ASCII characters work correctly.
• Missing, moved, or deleted files produce a clear non-destructive error.
• Only files allowed by the current workspace/sandbox boundary can be opened; transcript content must not bypass filesystem permissions.
• The conversation remains visible and preserves its scroll position while previewing a document.

This should reuse the existing right workbar and file-preview experience rather than introduce another competing document surface.

Alternatives or workarounds

Today, users copy the path from the conversation, manually locate the file in Finder/Explorer, and open it in a separate application. This works but is slow and disconnects the referenced document from the session context.

Related: #1031 introduced the resizable right session workbar and its Files preview surface.