[idea] Attach an existing tmux session as a workspace

[potto007]

idea / problem

I run herdr as my daily terminal on the desktop, and I run long jobs on rented GPU boxes and on that same desktop inside tmux.

requested change

• herdr tmux list - list the sessions on the tmux server, with the window count, whether a client is attached, the session path, and which herdr workspace already holds it, if any.
• herdr tmux attach <session> - open a workspace whose root pane is tmux attach-session on that session. Label it tmux:<session>, default the cwd to the session path, and if the session is already open in a workspace, focus that one instead of making a second. Detaching inside the pane should leave a shell rather than close the workspace, and a session restore should re-attach rather than come back as an empty shell.

That is the whole ask. I am not looking for herdr to manage tmux windows or panes, or to translate tmux keybinds. tmux keeps owning the session; herdr gets a window into it that shows up in the sidebar next to everything else.

why you want this

Because the alternative is two multiplexers that do not know about each other. With this change, the session has a name, survives a herdr restart, and can be read over the socket API like any other pane. I let agents watch long-running work through herdr pane read, and today anything living in tmux is invisible to them.

I have this working on a branch of my fork (potto007/herdr, feat/tmux-workspaces) with tests and draft docs, and it came out small because herdr already persists a per-pane launch argv. I have read the PR policy, so I am not opening one. If the idea fits, the branch is there to take or to ignore.