Keep a persistent cwt:worktrees explorer tab in tmux/zellij

[0dragosh]

Motivation

• Keep the worktrees navigator open as a persistent explorer tab/window so users can keep it alongside per-worktree tabs and switch worktrees without leaving the multiplexer. This should work for both zellij and tmux.

Description

• Add a shared constant WORKTREES_EXPLORER_NAME with value "cwt:worktrees" in src/tmux/pane.rs.
• Add ensure_worktrees_explorer_tab() in src/tmux/pane.rs which renames the current multiplexer container using zellij action rename-tab or tmux rename-window depending on backend.
• Call ensure_worktrees_explorer_tab() at startup from both interactive UIs in run_tui and run_forest_tui in src/main.rs when cwt is already inside a multiplexer, and treat rename failures as non-fatal warnings.

Testing

• Ran formatting with cargo fmt --all and it completed successfully.
• Built the project with cargo check and cargo clippy --all-targets --all-features with no errors or warnings blocking the change.
• Ran the test suite with cargo test and all unit and integration tests passed (no failures).

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Codex Task