[Linux] Session persistence + multi-project + manifest file watching

[2witstudios]

Overview

Implement session persistence, multi-project support, and direct manifest file watching (inotify) to match macOS app behavior.

Session Persistence

Match PPG CLI/PPG CLI/DashboardSession.swift:

• Persist terminal/agent entries and grid layouts to .ppg/dashboard-sessions.json
• SessionData { entries: Vec<TerminalEntry>, gridLayouts: HashMap<String, GridLayoutNode> }
• TerminalEntry { id, label, kind, parent_worktree_id, working_directory, command, tmux_target, session_id, grid_owner_entry_id, variant_id }
• Debounced writes (1 second) via glib::timeout_add_seconds
• Synchronous flush on app quit (connect_shutdown)
• Restore sessions on relaunch: reconnect to tmux windows, rebuild pane grids

Multi-Project Support

Match PPG CLI/PPG CLI/Models.swift (OpenProjects):

• Vec<ProjectContext> persisted to config dir
• Each project has its own manifest, dashboard session, and manifest watcher
• Sidebar shows all open projects' trees
• Ctrl+1-9 switches to project's first item
• "Open Project" action adds a project to the list
• "Close Project" removes from list

Manifest File Watching

Match PPG CLI/PPG CLI/SidebarViewController.swift (ManifestWatcher):

• Use gio::FileMonitor (wraps inotify on Linux) to watch .ppg/manifest.json
• On change: debounce 200ms, then re-read manifest, apply tree diff to sidebar
• Safety fallback: 3s polling timer in case inotify misses events (file recreated)
• Also receive WebSocket manifest_updated events as a complement

Prerequisite Checks

Match PPG CLI/PPG CLI/SetupViewController.swift:

• On launch, check ppg --version and tmux -V (tmux ≥ 3.5)
• If missing: show setup screen with install instructions
• ppg: npm install -g pure-point-guard
• tmux: sudo apt install tmux / sudo dnf install tmux

Acceptance Criteria

• Terminal entries + grid layouts persist to dashboard-sessions.json
• Sessions restore on app relaunch
• Multi-project: open/close projects, switch with Ctrl+1-9
• Manifest file watching via inotify with 200ms debounce
• 3s polling fallback for missed events
• Prerequisites check screen shows when ppg/tmux missing