F8: Drag file references from file tree into thread composer #11

Summary

Allow users to drag a file from the file tree panel and drop it onto the chat composer to insert an @-mention reference, integrating with the existing file-mention system.

Scope

• Make file tree items draggable via dnd-kit (useDraggable with data: { type: "file", path, kind })
• Add a drop zone on the chat composer area (useDroppable)
• On drop: insert @<relative-path> at cursor position in the composer
• Leverage existing file-mention resolution logic (already handles @-mention file refs)
• Drag overlay shows filename during drag

Interactions

• Drop onto composer: inserts @-mention at cursor
• Drop onto a collapsed Thread in sidebar: activate that Thread, open composer, insert mention
• Drag a directory: inserts directory path as mention
• Drop outside valid zones: cancel, no action
• Multiple file drag (if dnd-kit supports): insert each path on separate lines

Constraints

• Only files within the active Project's workspaceRoot can be dragged
• Inserted reference uses relative path from workspaceRoot, not absolute path
• Cross-project file mentions not supported

Key files to modify

• apps/web/src/components/FileTreePanel.tsx — add useDraggable to file items
• apps/web/src/components/Sidebar.tsx — extend DndContext to support file drag type
• apps/web/src/components/ — composer component: add useDroppable zone
• apps/web/src/composerDraftStore.ts — insert @-mention at cursor position

Dependencies

• F7 (File tree panel must exist first)

Acceptance criteria

• Can drag a file from file tree to composer
• @-mention reference inserted at cursor position
• Existing file-mention resolution picks up the reference
• Drag overlay shows filename
• Directory drag inserts directory path
• Drop outside valid zone cancels cleanly