Improve ZMX picker return and previews

[rrvsh]

Summary

• make Escape return to the current local or remote ZMX session while Ctrl-C still cancels to the outer shell
• verify the current session still exists before attaching so Escape cannot recreate a closed session
• prefetch bounded remote history previews as soon as each SSH host becomes reachable
• prioritize the current session, cache previews in the picker temporary directory, and refresh the active preview automatically

Validation

• fish -n fish/zmx.fish
• git diff --check
• real PTY checks for distinct Escape and Ctrl-C behavior
• isolated local return-to-session and missing-session safety checks
• isolated remote preview prefetch check without cursor movement
• nix develop -c just check
• rebuilt and activated Nemesis
• verified the active Fish configuration contains the new Escape and cached-preview behavior
• manually confirmed the behavior on Nemesis