fix(editor): skip kitty new-tab when no listen socket to avoid Ctrl+E hang

[FelixZhang]

Summary

Fixes #310 — Ctrl+E freezing the whole TUI when leaf runs under herdr (or any PTY multiplexer) with kitty 0.49.1 and no allow_remote_control.

Root Cause

When KITTY_PID is present, leaf attempts kitty @ launch to open the editor in a new tab. Under a PTY multiplexer, the kitty-cmd DCS response cannot pass back through, so kitty @ waits indefinitely (confirmed with timeout 8 kitty @ ls). LaunchStrategy::RunAndCheck blocks in Command::status(), freezing the TUI before the in-place fallback can run. kitty 0.48.2 failed fast, which is why this started with the 0.49.1 upgrade.

Fix

Only attempt the kitty @ new-tab launch when KITTY_LISTEN_ON is set (socket mode). Without it, kitty @ cannot work reliably, so leaf falls straight through to the proven in-place editor path. This is independent of kitty's remote-control behavior and multiplexer environment.

Testing

• Two new unit tests covering the KITTY_LISTEN_ON gate (present / absent)
• Existing kitty_tab_title_uses_filename test updated for the gate
• cargo test — 512 passed, 0 failed
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --release
• Manually verified in herdr + kitty 0.49.1: Ctrl+E opens nvim in place and returns cleanly

The second commit collapses a nested else if flagged by nightly clippy on 1.28.3; no behavior change.