Add ability to set boot layout.

README.md

@@ -4,7 +4,7 @@ A proof-of-concept workspace built on
 [libghostty-vt](https://github.com/ghostty-org/ghostty) and GTK4/libadwaita:
 **vertical tabs**, each holding a split tree of terminal and web panes, with
 **saved layouts** that open a whole arrangement — directories, scripts and all —
-in one go.
+in one go, and a **startup list** that opens the ones you always want.
 
 The sidebar holds the window controls, a new-tab button, one row per tab, and a
 settings gear at its foot — the layout Zen Browser uses for vertical tabs —
@@ -124,8 +124,8 @@ Pty.zig        openpt/fork/exec, controlling terminal setup
 script.zig     runs a command for `$(...)` in a layout's directory
 key.zig        GDK keyval -> libghostty-vt key mapping
 theme.zig      colors libghostty-vt has no opinion about, per scheme
-Settings.zig   preferences: model, JSON on disk
-SettingsDialog.zig    the settings page
+Settings.zig   preferences: theme and startup tabs, JSON on disk
+SettingsDialog.zig    the settings page, including the startup list editor
 TabSettingsDialog.zig one tab's own settings, and the emoji picker
 emoji.zig      generated: every emoji the picker offers, and the search over them
 ```
@@ -288,6 +288,55 @@ A node is a split if it has a `split` key and a leaf otherwise. Saves are
 atomic — written to a temporary and renamed — so an interrupted write leaves
 the previous layouts intact rather than a file that won't parse.
 
+## Startup tabs
+
+Opening the same three layouts against the same three worktrees every morning is
+the kind of thing the app should be doing for you. **Settings → Startup** is a
+list of tabs to open at launch: one row per tab, each naming a saved layout, the
+values to fill its parameters in with, and optionally a name and an emoji to pin
+on the row.
+
+The fastest way to fill it in is *Use current tabs*: arrange the window you want,
+then say that this is the window you always want. It writes down one entry per
+open tab, in sidebar order, with whatever each was opened with.
+
+What is saved is the **recipe, not the session**. An entry is the layout's name
+and a handful of values, so it is a few lines you can read, edit and keep — and
+the layout itself stays the thing you maintain. Nothing here restores a
+scrollback, a running command, or a shell that has since wandered into another
+directory; launching re-runs the arrangement, and that was the tedious part.
+
+Rows in the list can be reordered, and the first one is the tab you land in. An
+entry with no layout picked opens a plain shell, which is worth having — a
+startup list is often two configured tabs and one ordinary shell to work in. An
+entry naming a layout that has since been renamed or deleted is skipped with a
+warning rather than silently costing you a tab, and the settings page keeps
+showing it — with *This layout is no longer saved* under it — so the values you
+typed for it survive until you deal with it.
+
+Parameters that the entry doesn't mention open at the layout's own default, which
+is the same rule the parameter prompt follows. So an entry only has to carry the
+values that differ from what the layout already suggests.
+
+The list lives in `settings.json` beside the theme:
+
+```json
+{
+  "version": 1,
+  "theme": "dark",
+  "startup": [
+    { "layout": "Project", "name": "signal", "emoji": "🚀",
+      "parameters": { "path": "~/src/signal" } },
+    { "layout": "Project", "parameters": { "path": "~/src/playpen" } },
+    { "name": "scratch" }
+  ]
+}
+```
+
+With no `startup` list the app opens a single shell, exactly as it did before
+this existed — and if a list opens nothing at all, that is what you get too,
+since a window you can't type in is not an outcome worth being faithful for.
+
 Two design choices worth calling out:
 
 **No IO thread.** The PTY is read on the GLib main loop through a unix fd
@@ -324,6 +373,9 @@ in principle, but a terminal grid is small.
 - **Saved layouts**: whole tabs — panes, splits, ratios, per-pane directories
   and scripts — opened in one go, parameterised by `{{name}}`, authored by
   arranging a tab and saving it. See [Layouts](#layouts)
+- **A startup list**: the tabs to open at launch, each a layout with its
+  parameters filled in, fillable from the tabs you have open now. See
+  [Startup tabs](#startup-tabs)
 - **Pane status in the tab strip**, driven by OSC 9;4, so a tab can say whether
   it is working, waiting on you, or finished and still unanswered. See
   [Agent status](#agent-status)
@@ -566,16 +618,18 @@ the tab simply keeps the name it had.
 
 Deep navy surfaces with Signal's ultramarine (`#3a76f0`) as the accent, in a
 light and a dark scheme. `Ctrl+,` or the gear at the foot of the sidebar opens
-**Settings**, which currently holds one choice: light, dark, or **system**,
-which is the default and follows the desktop.
+**Settings**, whose Appearance section holds the choice: light, dark, or
+**system**, which is the default and follows the desktop.
 
 The preference lives in `~/.config/playpen/settings.json` (or
-`$XDG_CONFIG_HOME`), beside `layouts.json`:
+`$XDG_CONFIG_HOME`), beside `layouts.json`, alongside the
+[startup tabs](#startup-tabs):
 
 ```json
 {
   "version": 1,
-  "theme": "system"
+  "theme": "system",
+  "startup": []
 }
 ```
 
@@ -632,8 +686,10 @@ This is a proof of concept, and the following are deliberately absent:
   nothing else: no bookmarks, history, downloads, devtools, or find-in-page,
   and each pane uses WebKit's default context, so nothing is persisted between
   runs. Links that ask for a new window are ignored rather than opening a pane.
-- **Restoring a session.** Layouts save arrangements you open deliberately;
-  nothing restores the tabs you happened to have open when the app closed.
+- **Restoring a session.** [Startup tabs](#startup-tabs) reopen the
+  arrangement you asked for, but nothing captures the state a session got itself
+  into: no scrollback, no running commands, no directories a shell wandered off
+  to. A tab comes back as its layout, not as you left it.
 - **Kitty graphics, hyperlinks, tab reordering.**
 - **Custom terminfo.** `TERM` is reported as `xterm-256color` rather than
   `ghostty`, since we don't install a terminfo entry.