Update golangci-lint to v2.15.0 and fix the linter config

Convert .golangci.yml to the v2 layout: linters-settings and
issues.exclude-use-default were rejected by the v2 schema, so the lll,
funlen, cyclop, and dupl settings were never applied. Disable
exhaustruct_v5 (the newer major version of the already-disabled
exhaustruct) and the deprecated gomodguard. Pin the golangci-lint image
at v2.15.0 in the Makefile and Dockerfile.

.golangci.yml

@@ -9,24 +9,26 @@ linters:
   disable:
     # Genuinely incompatible with project patterns
     - exhaustruct      # Requires all struct fields
+    - exhaustruct_v5   # Same linter, newer major version (since v2.13.0)
     - depguard         # Dependency allow/block lists
     - godot            # Requires comments to end with periods
     - wsl              # Deprecated, replaced by wsl_v5
+    - gomodguard       # Deprecated, replaced by gomodguard_v2
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go
+  settings:
+    lll:
+      line-length: 88
+    funlen:
+      lines: 80
+      statements: 50
+    cyclop:
+      max-complexity: 15
+    dupl:
+      threshold: 100
 
-linters-settings:
-  lll:
-    line-length: 88
-  funlen:
-    lines: 80
-    statements: 50
-  cyclop:
-    max-complexity: 15
-  dupl:
-    threshold: 100
-
+# No default exclusions: v2 applies none unless linters.exclusions.presets
+# is set, which replaces v1's exclude-use-default: false.
 issues:
-  exclude-use-default: false
   max-issues-per-linter: 0
   max-same-issues: 0