build: update golangci-lint to v2.12.2 with canonical config

Add the canonical .golangci.yml (v2 schema, all linters enabled with a
small documented disable list) and pin the Dockerfile lint stage to
golangci/golangci-lint:v2.12.2 by tag and digest, replacing the old
v1.64.8 digest-only pin.

Fix all findings surfaced by the v1 to v2 jump without changing any
exported signatures or behavior:

- add package and exported-symbol doc comments (revive)
- rename unused handler parameters to underscore (revive)
- check or explicitly discard error returns (errcheck, errchkjson)
- wrap errors with %w instead of %v (err113)
- use http.NewRequestWithContext instead of http.Post (noctx)
- replace fmt.Println with fmt.Fprintln(os.Stdout, ...) (forbidigo)
- name magic numbers as constants (mnd)
- add explicit slog.LevelDebug case (exhaustive)
- interface{} to any (modernize)
- move tests to the simplelog_test package (testpackage) and add
  t.Parallel() (paralleltest)
- move NewWebhookHandler above its methods (funcorder)
- whitespace, line-length, and blank-line fixes (wsl_v5, whitespace,
  nlreturn, lll, embeddedstructfieldcheck)
- nolint with justification for the intentional init/global design
  (gochecknoinits, gochecknoglobals) and interface-returning
  constructor (ireturn)

.golangci.yml

@@ -0,0 +1,34 @@
+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.
+
+run:
+  timeout: 5m
+  modules-download-mode: readonly
+
+linters:
+  default: all
+  disable:
+    # Genuinely incompatible with project patterns
+    - exhaustruct      # Requires all struct fields
+    - depguard         # Dependency allow/block lists
+    - godot            # Requires comments to end with periods
+    - wsl              # Deprecated, replaced by wsl_v5
+    - 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
+
+issues:
+  max-issues-per-linter: 0
+  max-same-issues: 0