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 block list restates only decisions already recorded in the Go
package defaults: zerolog, the pre-fork go-redis/redis, sergi/go-diff
and hexops/gotextdiff. Recorded rejections that repos vendoring this
file still require are deliberately absent, so a re-vendor cannot
redden a repo that has done nothing wrong.

Model: opus-5

.golangci.yml

@@ -61,20 +61,15 @@ 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. An entry matches the module path exactly unless it
-    # sets a match-type.
+    # listed here. Entries match the module path exactly (the default
+    # match-type), so a differently-versioned module path is unaffected.
     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."