* Open chat file links in a side pane on cmd-click
* Add hover tooltips to file links showing cmd-click hint
* Collapse file-link open path to a single disposition seam
Replace the parallel "open in side" callback that was threaded
alongside the normal open callback at every layer between the chat
click handler and the workspace router. One callback now carries a
`OpenFileDisposition` ("main" | "side") so the modifier-key decision
lives at the leaf and the routing decision lives at the workspace
screen, with no branching in between.
* Give assistant file links a home
