Proposal: give a tab a git identity — one tab per worktree

[oliviermattei]

Opening this as a discussion rather than an issue, because it changes what a tab
means, and that call is yours.

Terminology below: tab is the sidebar row that owns a pane tree, pane is
one terminal inside it.

Why I am asking for this

How I actually work: I open a workspace by opening a folder, because I am opening
a project to work in it. So the workspace is a repo, and the tab it starts with
sits on that repo's default branch. From there I open several panes — usually
four or five — because on one branch I want several agents running at once plus a
dev server. That part works well today.

What does not work is what happens when a second worktree enters the picture.
A checkout has exactly one active branch at a time, but nothing in the UI ties a
pane to the worktree it was opened in. Two panes on two different worktrees sit
side by side in the same tab, visually identical. I have already run a command
in the wrong worktree because of this. It is not a cosmetic problem — it is a
class of mistake the app currently makes easy.

My ideal shape is therefore: one tab per worktree, several panes inside it.
Switching worktree becomes switching tab, which is a deliberate gesture with a
visible target, instead of guessing which of five identical panes belongs where.

What I am proposing, in substance

Bind a tab to a git worktree of its workspace.

• The tab carries the git identity. Bound, its sidebar row shows the
  worktree/branch name, and the diffstat becomes meaningful again — it is per-tab,
  not a workspace-wide constant. Every pane inside that tab is understood to live
  in that worktree.
• Creating a pane stays inside the current worktree. Wanting another worktree
  means wanting another tab, not another pane in this one.
• When an agent wants to create a worktree, it offers to create the tab that
  goes with it, so the two never drift apart.
• Unbound tabs keep today's behavior, unchanged. This is opt-in, not a new
  mandatory concept.

Something we discussed before that fits here: being able to split the window and
put two tabs side by side. With tabs carrying a worktree identity, that split
becomes the supported way to work on two branches at once — with a visual
separation that says which is which — instead of the accidental, unmarked mix I
get today inside a single tab.

Where this collides with the current code

Since 4f8c982 (the tab level between the workspace and the layout), a tab is
purely a layout container: a title plus a pane tree. Nothing says what it is
working on. The same commit dropped the git branch and the diffstat from the rail,
with this rationale (src-app/src/app/sidebar/mod.rs, render_workspace_meta_row):

Neither the git branch nor a diffstat is rendered in the rail: both crowded the
row for little signal, and the Diff view is where change counts belong.

That reasoning holds for a workspace row: one workspace, one checkout, one
branch — the line is nearly constant, so it is mostly noise. It stops holding the
moment a tab has its own worktree, because then the branch is exactly what tells
two tabs apart, and the diffstat is exactly what tells me which one has work in
it.

Open questions

1. Does a tab own its worktree (created and removed with the tab), or merely
   point at one that already exists? Owning it means removal semantics, and a
   removed worktree with uncommitted work is destructive.
2. Where does the binding live in paneflow.json? Session schema v2 already
   persists tabs, so a worktree field on the tab is the obvious spot, but that
   is a schema change with a migration.
3. Does the branch + diffstat line come back only for bound tabs, or does this
   reopen the density question in general? (Related: an Appearance setting for
   sidebar density, compact vs detailed, previewed the way theme presets already
   are.)
4. Interaction with the diff dock, which already has its own notion of what is
   being reviewed.

No code written for this — purely a design question, and it needs your call before
anything is built.