Make use of nushell's autoload directory

In some projects $NIX_CFLAGS_COMPILE now exceeds 10k. Maybe we should
make this configurable, but it also feels reasonable to just support
whatever the system supports. The maximum length appears to depend on
the platform -- PAGE_SIZE * 32 on Linux (usually 128 KiB), and capped by
ARG_MAX on macOS (256 KiB across all args). 64 KiB ought to be enough
for anyone.

Bump memory limit

This avoids the situation where shadowenv will fully resolve to a nix
store path which then isn't tracked by nix gc-roots.

… to avoid incorrect reordering of PATH

Replace __fish_reconstruct_path hook on first run of __shadowenv_hook to avoid incorrect reordering of PATH

Shopify/hookbook@b90c995
Shopify/hookbook@9d781b8

Vendor hookbook

When a shell locates `shadowenv` through $PATH, argv[0] is the bare name
"shadowenv". `shadowenv init <shell>` joined that name with the current
directory, so the generated hook ran `"$PWD/shadowenv" hook` on every
prompt and failed with "No such file or directory" from any directory
other than the one shadowenv happens to live in.

Resolve the executable path correctly instead:

  * absolute argv[0] is used as-is,
  * a relative path containing a separator (e.g. `./shadowenv`) is joined
    with the cwd, as before,
  * a bare name is looked up on $PATH.

We still avoid `current_exe()` for these cases so a stable symlink is
preferred over a canonicalized (and potentially garbage-collected)
target such as a Nix store path; it is only used as a last-resort
fallback, never `$PWD/<name>`.

Adds unit tests covering each case, including a regression test ensuring
a bare name is never resolved to `$PWD/shadowenv`.

Fix `init` resolving argv[0] to `$PWD/shadowenv` for bare invocations