Update golangci-lint to v2.12.2 with canonical config

- Add canonical .golangci.yml (v2 schema, default: all, project
  thresholds for lll/funlen/cyclop/dupl)
- Bump golangci-lint pins from v2.0.2 to v2.12.2 in Makefile
  (go install, new /v2 module path) and Dockerfile (tagged+digest
  Debian image pin)
- Fix all lint findings surfaced by the new linter set across
  cmd/mfer, internal/bork, internal/cli, internal/log, and mfer:
  static sentinel errors (err113), context-aware HTTP and exec
  (noctx), guarded integer conversions and stricter permissions
  (gosec), named constants (mnd, goconst), function decomposition
  (funlen, cyclop, gocognit, nestif), declaration ordering
  (funcorder), t.Parallel/t.TempDir/t.Setenv adoption in tests
  (paralleltest, usetesting), protobuf getters (protogetter), plus
  formatting and style cleanups (wsl_v5, nlreturn, lll, revive,
  testifylint, and others)
- Serialize CLI runs in tests behind a mutex so parallel tests do
  not cross-wire the process-global logger's captured output

.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