Resolve shell dynamically instead of hardcoding a NixOS store path

[yottanami]

Summary

explicit-shell-file-name was hardcoded to /run/current-system/sw/bin/fish in plugins/editor/core.el, a NixOS-only path, and fish isn't even in this package's own Nix closure (nix/package.nix's runtimeTools). Breaks M-x shell on any non-NixOS system, including a plain nix run elsewhere.

The rest of the package wires Nix-provided binaries in via PATH (nix/package.nix's makeWrapper + --prefix PATH), not baked-in store paths, so this resolves the shell the same idiomatic way instead of hardcoding a different path: prefer fish if it's actually on PATH (e.g. if someone adds it via extraRuntimeInputs), fall back to $SHELL (treating an empty-but-set $SHELL as unset, since that's non-nil/truthy in elisp), then bash, then /bin/sh.

Notes

Nix and Emacs are both available in this environment now, so this was actually run, not just read.

Test plan

• nix flake check --no-build: all checks pass.
• nix build .#noteditor: builds clean.
• Ran the built ./result/bin/noteditor --batch and printed explicit-shell-file-name after init: resolves to /bin/bash, matching this environment's $SHELL. Confirms the fix works in the actual packaged product, not just an isolated snippet.
• Ran the actual (or (executable-find "fish") ...) fallback chain in a real emacs --batch process across all four branches:
  • fish on PATH -> resolves to the fish store path.
  • normal $SHELL set, no fish -> resolves to $SHELL.
  • $SHELL="" (empty but set), no fish -> resolves to bash via executable-find (this caught a real bug in the first version of this fix: an empty string is non-nil in elisp, so a plain (getenv "SHELL") in the or chain accepted "" and skipped the bash/sh fallbacks entirely, leaving explicit-shell-file-name empty; fixed by treating empty as unset).
  • $SHELL truly unset, no fish -> resolves to bash via executable-find.
• grep -rn "/run/current-system" --include="*.el" .: no hardcoded NixOS store paths remain anywhere in the elisp tree.

Closes Men-in-Black-5/ideas#18

[yottanami]

Reviewed. The change itself looks correct and well-scoped:

• Balanced parens verified programmatically (not just eyeballed) across the whole file.
• fish genuinely isn't referenced anywhere else in the repo — matches the "not in this package's Nix closure" claim.
• No /run/current-system/... paths remain anywhere in the .el tree (satisfies that acceptance criterion).
• The (or (executable-find "fish") (getenv "SHELL") (executable-find "bash") "/bin/sh") fallback chain is idiomatic and matches how the rest of the package wires Nix-provided tools in via PATH (cross-checked against nix/package.nix's makeWrapper --prefix PATH usage) rather than baked-in store paths.

That said, this is held rather than merged: no agent has nix/emacs available to actually run nix flake check or load the config, same as the PR itself flags. Per policy, changes that no agent can actually run/verify wait for the owner's explicit confirmation before merging, not static review alone — @yottanami, once you've loaded this (or run nix flake check) and confirmed M-x shell behaves, I'll merge.