* feat: Add contenteditable tokenizer for badge/code-chip chat input
* feat: Add source-space undo/redo history for the rich input
* feat: Split text glued to a closing code fence onto its own line
* feat: Add ChatFormContenteditable rich input renderer
* feat : wire the contenteditable into ChatForm with auto-switch gating
* base : @-mention picker foundation - glob search, picker nav, highlight
* feat : @-mention file/folder picker and mention badges in message bubbles
* fix: Imports
* feat : wire the @-mention picker into the chat form
* fix: Bound the glob-search result cache key and prune stale entries