Migrate canonical .golangci.yml to gomodguard_v2 with a block list (closes #25)

golangci-lint v2.12.0 deprecated gomodguard, and `linters.default: all`
enables it, so every lint run in every consuming Go repo prints a
deprecation warning. Disable the old name, which is what silences the
warning; name gomodguard_v2 in linters.enable so the settings block has
a visible owner; give it a blocked module list.

The list restates only decisions already recorded in the Go package
defaults: zerolog, the pre-fork go-redis/redis, sergi/go-diff and
hexops/gotextdiff. The go-redis entry matches by prefix, because the
pre-fork module's later releases are separate module paths that an
exact match would let through; the successor sits under a different
prefix. Recorded rejections that vendoring repos still require are
deliberately absent, so a re-vendor cannot redden a repo that has done
nothing wrong.

Model: opus-5

.golangci.yml

@@ -61,15 +61,20 @@ linters:
                 Test-support code belongs in test files and in packages whose
                 directory name ends in test, not in the shipped binary.
     # Only decisions already recorded in the Go package defaults are
-    # listed here. Entries match the module path exactly (the default
+    # listed here. An entry matches the module path exactly unless it
-    # match-type), so a differently-versioned module path is unaffected.
+    # sets a match-type.
     gomodguard_v2:
       blocked:
         - module: github.com/rs/zerolog
           recommendations:
             - log/slog
           reason: "Structured logging is stdlib log/slog."
+        # Prefix, because the pre-fork module's later releases are
+        # separate module paths (.../redis/v7, .../redis/v8) that an
+        # exact match would let through. The successor is under
+        # github.com/redis/go-redis/, which this prefix does not cover.
         - module: github.com/go-redis/redis
+          match-type: prefix
           recommendations:
             - github.com/redis/go-redis/v9
           reason: "Pre-fork module; use the maintained go-redis v9."