[BUG] --tmux should not require --worktree — breaks multi-worktree session workflows

Description

Running claude --tmux in an existing working directory (without --worktree) fails:

Error: --tmux requires --worktree

Workflow Context

My workflow involves multiple active worktrees across 2–3 terminals (e.g. inside Cursor IDE). I progress through issues by creating worktrees one at a time from a single session — I do not exit Claude between them.

The --tmux flag is valuable to me as a session resilience mechanism — if one terminal crashes (e.g. Cursor's integrated terminal drops), I want to reattach to the running Claude session via tmux. This is entirely independent of worktree creation.

Expected Behavior

--tmux should work without --worktree. These are orthogonal concerns:

• --worktree = create/enter a git worktree
• --tmux = manage the session via tmux (resilience / reattach)

Coupling them forces users to create an unnecessary worktree just to get tmux session management, which is wrong for workflows that operate on an existing working directory.

Actual Behavior

Error: --tmux requires --worktree

Suggested Fix

Remove the hard dependency. Allow --tmux to be used standalone so users can get tmux-managed sessions regardless of whether a worktree is involved.