Update the canonical .golangci.yml for golangci-lint v2.15.0 (closes #142)

Moves the pin to v2.15.0 and enables canonicalheader again, because that
release fixes the random misses behind
#105.

The depguard exemption reached every directory below one ending in test,
and a whole checkout under such a directory. It now covers only the files
directly inside. gomodguard_v2 also blocks github.com/go-redis/redis/v9,
whose pre-releases can still be imported.

Three comments are corrected. A new comment says the file has no
formatters section, so golangci-lint run checks no formatting.

Model: opus-5-5

.golangci.yml

@@ -3,6 +3,9 @@ version: "2"
 # Config schema uses the golangci-lint v2 layout (settings live under
 # linters.settings, not top-level linters-settings) so that the
 # thresholds below are actually applied by golangci-lint >= v2.
+#
+# This file has no formatters section, so golangci-lint run checks no
+# formatting. A repository's script/fmt-check checks it, on the host.
 
 run:
   timeout: 5m
@@ -16,17 +19,15 @@ linters:
     - gomodguard_v2
   disable:
     # Genuinely incompatible with project patterns
-    - exhaustruct      # Requires all struct fields
-    - exhaustruct_v5   # Requires all struct fields (successor to exhaustruct)
+    - exhaustruct_v5   # Requires all struct fields
     - godot            # Requires comments to end with periods
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go
     # Deprecated: the warning is attached to the old name, so it is
     # silenced by disabling that name, not by enabling the successor.
+    - exhaustruct      # Deprecated, replaced by exhaustruct_v5
     - wsl              # Deprecated, replaced by wsl_v5
     - gomodguard       # Deprecated, replaced by gomodguard_v2
-    # Misses findings at random in v2.14.0; back once a pinned release fixes it
-    - canonicalheader
   settings:
     lll:
       line-length: 88
@@ -42,7 +43,8 @@ linters:
       # test-support package exists to hand a test privileges the program
       # itself must never have, so a file that is not a test must not import
       # one. Test files, and the files inside a package whose directory name
-      # ends in `test`, are where that code belongs, and are exempt.
+      # ends in `test`, are where that code belongs, and are exempt. A package
+      # in a subdirectory of that directory is not exempt.
       #
       # The deny list below is the one part of this file a repository is
       # expected to extend, and the only part it may. depguard matches an
@@ -57,14 +59,14 @@ linters:
           files:
             - "$all"
             - "!$test"
-            - "!**/*test/**"
+            - "!**/*test/*"
           deny:
             - pkg: net/http/httptest
               desc: >-
                 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. Every entry matches the module path exactly.
+    # Each entry names a module not to use and what to use instead. Every
+    # entry matches the module path exactly.
     gomodguard_v2:
       blocked:
         - module: github.com/rs/zerolog
@@ -73,7 +75,7 @@ linters:
           reason: "Structured logging is stdlib log/slog."
         # One entry per pre-fork module path, because the later releases
         # are separate paths. A prefix match would be shorter but would
-        # also reach github.com/go-redis/redismock, the test double for
+        # also reach github.com/go-redis/redismock/v9, the test double for
         # the successor these entries recommend.
         - module: github.com/go-redis/redis
           recommendations:
@@ -87,6 +89,10 @@ linters:
           recommendations:
             - github.com/redis/go-redis/v9
           reason: "Pre-fork module; use the maintained go-redis v9."
+        - module: github.com/go-redis/redis/v9
+          recommendations:
+            - github.com/redis/go-redis/v9
+          reason: "Pre-fork module; use the maintained go-redis v9."
         - module: github.com/sergi/go-diff
           recommendations:
             - github.com/aymanbagabas/go-udiff