fix(guix): restore MPL-2.0 (squisher clobber downgraded it to PMPL) (#51)

`guix.scm` declared `PMPL-1.0-or-later` while its **own SPDX header said
MPL-2.0** and the root `LICENSE` is the Mozilla Public License — the
file contradicted itself. That is the squisher-corpus clobber signature.

Measured across all 418 estate repos: **64** guix.scm files asserted
PMPL; exactly **2 were self-consistent** and are deliberately untouched
(`palimpsest-license`, `polystack`). The other **62**, including this
one, were contradictory.

Now uses Guix's built-in `mpl2.0` instead of the hand-rolled licence
record, which also drops the palimpsest-license URL that had no business
on an MPL-2.0 package.

**Verified:** no PMPL remains; file still parses as Scheme.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: hyperpolymath

guix.scm

@@ -14,5 +14,4 @@
   (synopsis "metadata-grammar")
   (description "metadata-grammar — part of the hyperpolymath ecosystem.")
   (home-page "https://github.com/hyperpolymath/metadata-grammar")
-  (license ((@@ (guix licenses) license) "PMPL-1.0-or-later"
-             "https://github.com/hyperpolymath/palimpsest-license")))
+  (license mpl2.0))