[devbox shell] ensure all zsh startup files are linked to ZDOTDIR (#250)

## Summary

User `mtunski` in discord helpfully pointed out that their shell setup
using
zsh4humans (https://github.com/romkatv/zsh4humans) wasn't working with
devbox shell.

Specifically, they get an error saying:
> Installing nix packages. This may take a while...done.
Starting a devbox shell...
/tmp/devbox3134336657/.zshrc:62: command not found: z4h

We currently just copy over the `.zshrc` file into the new `ZDOTDIR` of
devbox shell. The fix is:
1. to link more zsh startup files (specifically, .zshenv, .zprofile,
.zlogin)
2. manually link other zsh-plugin specific files in the .zshrc. This is
not great, but a workaround until we introduce a more formal
architecture for custom shells to be hooked up (i.e. with a provider or
plugin model).



## How was it tested?

1. installed zsh4humans on my laptop
2. 
The advanced config docs point to three files to manage for [Backup and
Restore](https://github.com/romkatv/zsh4humans/blob/master/tips.md#backup-and-restore)
for zsh4humans:
- zshenv
- zshrc
- p10k*.zsh

The first two are handled by devbox CLI as part of zsh startup scripts.
The last one needs to be manually handled.

Added the following to my .zshrc:
```
# within my .zshrc file:
...
#### my addition ###
if [ -n "${ZDOTDIR+1}" ] && [[ $HOME != $ZDOTDIR ]] then
  ln $HOME/.p10k*.zsh $ZDOTDIR
fi
#### end of my addition ###

# Install or update core components (fzf, zsh-autosuggestions, etc.) and
# initialize Zsh. After this point console I/O is unavailable until Zsh
# is fully initialized. Everything that requires user interaction or can
# perform network I/O must be done above. Everything else is best done below.
z4h init || return
```
3. opened `devbox shell` in `testdata/zig/zig-hello-world`. Did `which
zig`
to verify that the nix paths are active (i.e. devbox shell is active).

NOTE: in the absence of step (2), the `z4h init` step will start a setup
wizard.

4. Opened a new zsh shell (new terminal window) to ensure the .zshrc
changes are okay.

4. Deleted zsh4humans (instructions
https://github.com/romkatv/zsh4humans#uninstalling), and re-did step 3
without issues.

Author: savil

nix/shell.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 	_ "embed"
 	"fmt"
+	"io/fs"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -202,6 +203,9 @@ func (s *Shell) execCommand() string {
 		return strings.Join(append(args, s.binPath), " ")
 	}
 
+	// Link other files that affect the shell settings and environments.
+	s.linkShellStartupFiles(filepath.Dir(shellrc))
+
 	// Shells have different ways of overriding the shellrc, so we need to
 	// look at the name to know which env vars or args to set.
 	var (
@@ -286,6 +290,38 @@ func (s *Shell) writeDevboxShellrc() (path string, err error) {
 	return path, nil
 }
 
+// linkShellStartupFiles will link files used by the shell for initialization.
+// We choose to link instead of copy so that changes made outside can be reflected
+// within the devbox shell.
+//
+// We do not link the .{shell}rc files, since devbox modifies them. See writeDevboxShellrc
+func (s *Shell) linkShellStartupFiles(shellSettingsDir string) {
+
+	// For now, we only need to do this for zsh shell
+	if s.name == shZsh {
+		// Useful explanation of zsh startup files: https://zsh.sourceforge.io/FAQ/zshfaq03.html#l20
+		filenames := []string{".zshenv", ".zprofile", ".zlogin"}
+		for _, filename := range filenames {
+			fileOld := filepath.Join(filepath.Dir(s.userShellrcPath), filename)
+			if _, err := os.Stat(fileOld); errors.Is(err, fs.ErrNotExist) {
+				// this file may not be relevant for the user's setup.
+				continue
+			} else if err != nil {
+				debug.Log("os.Stat error for %s is %v", fileOld, err)
+			}
+
+			fileNew := filepath.Join(shellSettingsDir, filename)
+
+			if err := os.Link(fileOld, fileNew); err == nil {
+				debug.Log("Linked shell startup file %s to %s", fileOld, fileNew)
+			} else {
+				// This is a best-effort operation. If there's an error then log it for visibility but continue.
+				debug.Log("Error linking zsh setting file from %s to %s: %v", fileOld, fileNew, err)
+			}
+		}
+	}
+}
+
 // envToKeep is the set of environment variables that we want to copy verbatim
 // to the new devbox shell.
 var envToKeep = map[string]bool{