fix(php plugin): reference php output via explicit flake fragment

The php plugin's generated flake exports two packages (php and composer),
but referenced php via the bare `path:{{ .Virtenv }}/flake` (the flake's
`default` output) while composer used an explicit `#composer` fragment.

Devbox's own flake example documents that flakes exporting more than one
package must be referenced with explicit url fragments
(e.g. `path:.../flake#pkg`); the bare `default` reference is intended for
single-output flakes. Relying on the bare default for one of two outputs is
fragile and has been reported to result in the `php` binary not being linked
into the environment (only `composer` appears).

Add a named `php` output to the plugin flake (keeping `default` for
backwards compatibility) and reference it explicitly as
`path:.../flake#php`, mirroring the working `#composer` entry. Bump the
plugin version so cached create_files are regenerated.

Fixes #2788

Author: claude

plugins/php.json

@@ -1,9 +1,9 @@
 {
   "name": "php",
-  "version": "0.0.3",
+  "version": "0.0.4",
   "description": "PHP is compiled with default extensions. If you would like to use non-default extensions you can add them with devbox add php81Extensions.{extension} . For example, for the memcache extension you can do `devbox add php81Extensions.memcached`.",
   "packages": [
-    "path:{{ .Virtenv }}/flake",
+    "path:{{ .Virtenv }}/flake#php",
     "path:{{ .Virtenv }}/flake#composer"
   ],
   "__remove_trigger_package": true,

plugins/php/flake.nix

@@ -21,7 +21,12 @@
     in
     {
       packages.{{ .System }} = {
+        # This flake exports more than one package (php and composer), so each
+        # must be referenced from the plugin via an explicit url fragment
+        # (e.g. path:.../flake#php). The bare `default` output is kept for
+        # backwards compatibility with anyone referencing the flake directly.
         default = php;
+        php = php;
         composer = php.packages.composer;
       };
     };