feat: add tabs + panes hybrid layout support

[minorole]

Summary

• Add ability to create multiple tabs where each tab has its own pane layout
• Users configure tabs: N alongside any layout to get N tabs, each with the specified pane configuration
• Example: tabs: 3 + layout: duo = 3 tabs × 2 panes = 6 workspaces

Changes

• Config parsing: Add tabs: key support in lib/config.zsh
• New AppleScript: layout-hybrid.applescript creates tabs with splits in each
• Setup wizard: Now asks "Do you want multiple tabs?" before layout selection
• Terminology: Renamed "panes" to "sections" in wizard for beginners
• Dry-run: Shows tabs × panes breakdown

Breaking Change

layout: tabs is removed. Users with this config will see:

Error: 'layout: tabs' is deprecated.

Update your config to use the new tabs + panes system:
  layout: 1        # panes per tab (or duo, trio, quad, etc.)
  tabs: N          # number of tabs (2-10)

Example - 3 tabs, each with 2 panes:
  layout: duo
  tabs: 3

Test plan

• Run gpane setup and choose "yes" for multiple tabs
• Run gpane myproject --dry-run with tabs: 3 + layout: duo config
• Test old layout: tabs config shows migration error
• Test per-project override with different tabs count
• Manual test: verify Ghostty creates correct tabs × panes structure

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