Scratchpad tabs in the workspace tab strip

[jacodv]

What problem does this solve?

When working in a worktree I often need somewhere to jot quick notes — a command I'm about to run, a snippet an agent produced, a checklist for the current task. Today the only options are opening an external editor/notes app (losing context and window focus) or abusing a terminal tab (e.g. cat > /dev/null or an editor session), which burns a Ghostty surface and a zmx session for what is just plain text.

Proposed solution

Add a "scratchpad" as a new tab kind alongside terminal tabs in the workspace tab strip: a full-pane plain-text editor (NSTextView with undo, find bar, and standard macOS text features) with no Ghostty surface, split tree, or zmx session. Creatable from a button next to "+" in the tab bar, the File menu, or a rebindable New Scratchpad shortcut (default ⌥⌘T). Contents persist per-worktree in the existing layout snapshot so notes survive app restarts, and legacy layouts decode safely.

Alternatives considered

• External notes apps / editors: lose window context and per-worktree association.
• A terminal tab running an editor: costs a surface + zmx session and conflicts with agent presence tracking.

Supacode version

Built from source (main @ c4e28e2)

Are you planning to build this yourself?

• I intend to open a pull request once this is approved and marked ready.

Before submitting

• I searched existing issues and this is not a duplicate.
• I understand feature pull requests are closed until the issue is marked ready.
• I agree to follow this project's Code of Conduct.

[github-actions]

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Please do not start any work or open a pull request until this issue is labeled ready. That label is a maintainer's go-ahead: a pull request opened against an issue that is not yet ready is labeled invalid until then. You will get a comment here the moment it is marked ready.

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[jacodv]

One note: I tried running the installed app and a debug instance side-by-side. This causes a lot of issues. Fabel suggested some fixes which I did not include in this PR.

[sbertix]

We're considering introducing multiple types of "panels" in the split manager (#641, #500, etc). If we add this, the implementation will most likely be at the surface level, not just the tab level. However, this is not planned for now.