feat: Add zellij-backed multiplexer support with tmux fallback

[0dragosh]

Motivation

• Add first-class support for the Zellij terminal multiplexer so cwt can run sessions under either zellij or tmux and behave the same in both environments.
• Prefer zellij when it is available while preserving existing tmux behavior and APIs so stored pane/session identifiers and higher-level logic remain compatible.

Description

• Implemented a multiplexer abstraction in src/tmux/pane.rs with a Multiplexer selector and preferred_multiplexer() that prefers zellij when present and falls back to tmux otherwise.
• Added zellij helpers (zellij_action, create_zellij_pane, tab name encoding/decoding helpers) and adapted create_pane, focus_pane, kill_pane, pane_exists, pane_current_command, list_panes, pane_pid, and send_keys to dispatch to the appropriate backend.
• Updated CLI/startup logic in src/main.rs so tmux auto-bootstrap is skipped when zellij is installed, replaced tmux-specific diagnostics with generic multiplexer wording, and extended related unit tests to cover the zellij-available path.
• Reworded session-launch error messages in src/session/launcher.rs and src/orchestration/dispatch.rs to refer to an active multiplexer (zellij preferred, tmux fallback) without changing higher-level session flow.

Testing

• Ran cargo fmt which completed successfully.
• Ran cargo check and cargo clippy --all-targets --all-features which completed without errors.
• Ran cargo test and all automated tests passed: unit tests 77 passed and integration tests 40 passed (total 117 passed, 0 failed).

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Codex Task