#510 Add zmx-backed workspace/session persistence

Summary

This PR implements session persistence in cmux by wiring workspace restore/save to zmx-backed Ghostty surfaces.

At a high level:

• terminal processes are owned by zmx sessions (not tied to cmux process lifetime)
• cmux persists workspace layout/state metadata and zmx session mappings
• on relaunch, cmux restores layout and reconnects to existing zmx sessions when available

Why This Approach

Persistence has two independent concerns:

1. Process lifetime persistence (shell/session survives app exit)
2. UI/layout persistence (tabs, panes, focused pane, metadata restore)

This implementation intentionally splits them:

• zmx handles durable terminal session lifetime and reattach
• cmux handles workspace structure + per-panel metadata snapshots

That separation keeps each layer focused and avoids embedding process supervision logic into cmux UI state management.

What Changed

1) zmx-backed terminal wiring in cmux

• TerminalSurface now passes zmx_session/zmx_create into Ghostty surface config.
• TerminalPanel constructors accept zmx session parameters.
• New app settings for session persistence:
  • Enable zmx Session Persistence
  • Kill Sessions on Workspace Close

2) Workspace snapshot + restore model

• Added codable snapshot model in WorkspaceStatePersistence.swift.
• Persisted data includes:
  • workspace list + selected workspace
  • split tree + divider positions
  • focused pane index
  • panel metadata (type, directory, custom title, pinned, browser URL)
  • per-terminal zmx session names
• Snapshot is written to:
  • ~/Library/Application Support/cmux/workspace-state.json

3) Deterministic zmx session naming + lifecycle

• Added stable workspace ID + deterministic naming:
  • cmux-<stable-id-prefix>-<panel-index>
• Session mapping is assigned across all relevant creation paths:
  • initial workspace terminal
  • new tab terminals
  • split terminals (including auto-created split path)
  • replacement terminal after closing last panel
• Rehydrates panel index counter on restore to prevent collisions.

4) Restore behavior and resilience

• Restore reconstructs split tree and panel metadata.
• For restored terminal panels:
  • if session exists/alive => attach (zmx_create = false)
  • if missing => recreate (zmx_create = true)
• If zmx is unavailable at launch:
  • layout still restores
  • terminals start fresh (non-zmx reattach path)

5) Mapping consistency fixes

• zmx mappings are removed on panel close / pane close.
• Detached surface transfer now carries zmx session mapping and restores it on attach.
• Restore flow force-closes preexisting tabs before reconstruction to avoid stale residual tabs/metadata.
• Panel directories are rehydrated from snapshot during restore.

6) Submodule/fork wiring

• .gitmodules points Ghostty submodule to mdsakalu/ghostty fork branch containing zmx backend support.
• ghostty.h synced with new zmx fields used by cmux.

Tests / Validation

• Added regression tests in cmuxTests/CmuxWebViewKeyEquivalentTests.swift covering:
  • initial/replacement panel zmx mapping lifecycle
  • detach/reattach zmx mapping preservation
  • restore path directory rehydration
• Local validation:
  • Debug build succeeds via xcodebuild
  • target test set for WorkspaceZmxPersistenceRegressionTests passes

Dependency

• Depends on: https://github.com/manaflow-ai/ghostty/pull/2

Related Issues

• Addresses: #480
• Follow-up to closed tracking issue: #143

Summary by CodeRabbit

• New Features

  • Terminal session persistence via zmx with deterministic per-panel names and automatic restoration
  • Settings UI to enable persistence and to kill sessions on workspace close (localized strings added)
  • zmx availability checks and session management (probe/kill) exposed
  • New action to copy a tab/window title to the clipboard

• Improvements

  • More robust initial presentation and retry behavior for terminals
  • Prevents zmx environment leakage on startup; closes/kills zmx on workspace/window close

• Chores

  • Updated submodule configuration