Start chat from a chosen directory

[mcintyre94]

Summary

• Directory selector in empty chat state — a tappable folder row above session suggestions lets you change the working directory per-chat without visiting Overview; selecting a directory immediately refreshes the session list for that path
• Hierarchical directory picker — DirectoryPickerView now drills down into subdirectories with a "Use" button to confirm selection; worktrees under ~/.wisp/worktrees/ appear in a dedicated section with {repo} / {branch} display names and session counts; worktrees load once on appear rather than re-scanning on every navigation step
• Fix session suppression — removed spriteUsesWorktrees; usesWorktree is now just worktreePath != nil, so non-worktree chats on a sprite that also has worktree chats correctly show their session suggestions (e.g. resuming from a git repo root)
• changeWorkingDirectory on ChatViewModel — persists the per-chat directory to SwiftData and re-fetches remote sessions; extracted clearRemoteSessions() helper and fixed selectRemoteSession not resetting hasAnyRemoteSessions
• String.displayPath extension — consolidates the /home/sprite → ~/ conversion previously duplicated across DirectoryPickerView, ChatView, and SpriteOverviewView

Test plan

• New chat: folder row appears above session suggestions showing current working directory
• Tap folder row → directory picker opens as sheet starting at parent of current directory
• Navigate into subdirectories; "Use" button selects currentPath; "Parent directory" row goes up; stops at /
• Worktrees section appears when ~/.wisp/worktrees/ has entries; shows {repo} / {branch} with session count
• Selecting a directory closes the sheet and session suggestions refresh for the new path
• On a sprite with worktree chats, open a non-worktree chat — session suggestions now appear (previously suppressed)
• Overview working directory picker still works via NavigationLink with no changes needed

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