Add "New Chat in Same Worktree" option

[mcintyre94]

Summary

• Long-pressing the new chat button (toolbar on iPhone/iPad, nav panel sidebar, chat switcher sheet) shows a context menu with a "New Chat in '[branch]'" option when the current chat has a worktree
• Tap still creates a normal new chat (no behaviour change for the common case)
• New chat inherits worktreePath, worktreeBranch, and workingDirectory from the source chat — starts fresh with no session ID, sharing the same git directory

Test plan

• Open a sprite that has at least one chat with a worktree (worktree-per-chat enabled, send a message)
• Long-press the square.and.pencil toolbar button — confirm context menu appears with "New Chat in '[branch]'" and "New Chat" options
• Tap "New Chat in Same Worktree" — confirm new chat opens with the same working directory as the source chat
• Confirm normal tap on the button still creates a plain new chat
• On iPad/Mac: long-press the "New Chat" entry in the sidebar nav panel — same menu should appear
• On iPhone: open chat switcher, long-press "+" — same menu should appear
• For a chat without a worktree, confirm no context menu appears (button behaves as before)

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