[new feature] Open Finder-dropped Markdown files as one-time previews

[jsgrrchg]

Context

A user expected to drag a Markdown file from Finder into the workspace and have the app open it for a one-off preview. The current external drag/drop path reaches the workspace, but openDroppedVaultPathsAtTarget only opens paths that already exist in vaultStore.entries, so files outside the vault are silently ignored.

Proposed behavior

• Dropping a supported external file from Finder onto an editor pane opens a temporary, read-only preview tab.
• The file is not copied into the vault.
• The file is not added to the file tree.
• The tab is not persisted/restored across sessions.
• The tab does not participate in autosave, dirty state, inline review, agent edited-files, or accept/reject change flows.
• If the dropped file is already inside the vault, keep the existing behavior and open it as a normal vault-backed tab.
• If the drop lands on a file tree folder, keep the existing behavior and copy the external file into that folder.

Initial scope

Support Markdown/text-like files first, especially .md, .mdx, .txt, and other safe text formats. The header can identify the tab as External Preview and expose Open Externally / Reveal in Finder actions.

Implementation notes

• Model this explicitly as a temporary/read-only external preview instead of treating it as a normal editable vault file.
• Avoid routing external preview tabs through useEditableFileResource, because that flow assumes relativePath is vault-scoped and saves via save_vault_file.
• Add a small backend read command for external preview metadata/content, with file existence/type validation and size limits.

Edge cases to cover

• External file dropped onto workspace pane opens a preview.
• External file dropped onto file tree folder copies into the vault instead.
• Vault file dragged from Finder opens as a normal vault tab.
• Multiple dropped files preserve order and insert consecutive tabs.
• Pane-edge drops create/use split panes correctly.
• Large text files are truncated or rejected without freezing the UI.
• Unsupported binaries/directories do not get read as text.
• Deleted/moved external files fail gracefully for Open Externally and Reveal in Finder.
• External previews are excluded from session restore, review state, autosave, and agent change tracking.