Summary

• Add terminal layout persistence: tabs, splits, and per-pane working directories are saved to layouts.json on app quit and worktree removal
• Restore layouts automatically when reopening a worktree, gated behind a new Restore Terminal Layout toggle in Appearance settings
• Layouts are always persisted regardless of the toggle, so enabling it can retroactively restore the most recent session — but only if the worktree hasn't been loaded yet or its terminal state has no open tabs

Details

• New TerminalLayoutSnapshot model captures the full tab/split tree including split ratios, tab metadata (title, icon, tint color), and focused pane index
• LayoutsPersistenceKey provides a @Shared persistence key backed by layouts.json
• WorktreeTerminalManager gains saveLayoutSnapshot / loadLayoutSnapshot closures wired in supacodeApp.swift
• Snapshots are saved on worktree selection change, worktree pruning, and applicationWillTerminate
• Restoration skips setup scripts and gracefully handles partial failures (logs warnings, falls back to default tab)
• SplitTree.inserting now accepts an optional ratio parameter to preserve split proportions
• TerminalTabID and TerminalTabTintColor gain Codable conformance

Test plan

• TerminalLayoutSnapshotTests — round-trip encoding/decoding, leaf count, first leaf extraction
• WorktreeTerminalManagerTests — snapshot capture and persistence on selection change
• SettingsFeatureTests — toggle persists to settings file
• Manual: open multiple tabs/splits, quit, relaunch — verify layout is restored
• Manual: toggle setting off, quit, relaunch — verify a single default tab opens
• Manual: toggle setting on after a previous session — verify retroactive restoration

Closes #204