fix: resolve all golangci-lint errors for stricter linter config

- Replace interface{} with any and fix value receiver mutation in log method
- Use fmt.Fprintln instead of forbidden fmt.Println in Exec output
- Fix appendAssign by splitting append and assignment into two lines
- Replace assert.Nil/NotNil with assert.NoError/Error for error checks
- Upgrade error assertions to require when followed by dependent assertions
- Replace os.Setenv with t.Setenv in all test functions
- Restructure TestFindEnvs to use t.Setenv with unique prefixes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.golangci.yaml

@@ -1,35 +1,89 @@
 version: "2"
+output:
+  sort-order:
+    - file
 linters:
+  default: none
   enable:
-    - asciicheck
-    - durationcheck
-    - errorlint
-    - gosec
-    - misspell
+    - bidichk
+    - bodyclose
+    - depguard
+    - errcheck
+    - forbidigo
+    - gocheckcompilerdirectives
+    - gocritic
+    - govet
+    - ineffassign
+    - mirror
+    - modernize
     - nakedret
-    - nilerr
+    - nilnil
     - nolintlint
     - perfsprint
     - revive
+    - staticcheck
+    - testifylint
+    - unconvert
+    - unparam
+    - unused
     - usestdlibvars
+    - usetesting
     - wastedassign
   settings:
-    gosec:
-      includes:
-        - G102
-        - G106
-        - G108
-        - G109
-        - G111
-        - G112
-        - G201
-        - G203
+    depguard:
+      rules:
+        main:
+          deny:
+            - pkg: io/ioutil
+              desc: use os or io instead
+            - pkg: golang.org/x/exp
+              desc: it's experimental and unreliable
+            - pkg: github.com/pkg/errors
+              desc: use builtin errors package instead
+    nolintlint:
+      allow-unused: false
+      require-explanation: true
+      require-specific: true
+    gocritic:
+      enabled-checks:
+        - equalFold
+      disabled-checks: []
+    revive:
+      severity: error
+      rules:
+        - name: blank-imports
+        - name: constant-logical-expr
+        - name: context-as-argument
+        - name: context-keys-type
+        - name: dot-imports
+        - name: empty-lines
+        - name: error-return
+        - name: error-strings
+        - name: exported
+        - name: identical-branches
+        - name: if-return
+        - name: increment-decrement
+        - name: modifies-value-receiver
+        - name: package-comments
+        - name: redefines-builtin-id
+        - name: superfluous-else
+        - name: time-naming
+        - name: unexported-return
+        - name: var-declaration
+        - name: var-naming
+          disabled: true
+    staticcheck:
+      checks:
+        - all
+    testifylint: {}
+    usetesting:
+      os-temp-dir: true
     perfsprint:
-      int-conversion: true
-      err-error: true
-      errorf: true
-      sprintf1: true
-      strconcat: true
+      concat-loop: false
+    govet:
+      enable:
+        - nilness
+        - unusedwrite
   exclusions:
     generated: lax
     presets:
@@ -37,19 +91,24 @@ linters:
       - common-false-positives
       - legacy
       - std-error-handling
-    paths:
-      - third_party$
-      - builtin$
-      - examples$
+    rules:
+      - linters:
+          - errcheck
+          - staticcheck
+          - unparam
+        path: _test\.go
+issues:
+  max-issues-per-linter: 0
+  max-same-issues: 0
 formatters:
   enable:
-    - gci
     - gofmt
-    - goimports
+    - gofumpt
     - golines
+  settings:
+    gofumpt:
+      extra-rules: true
   exclusions:
     generated: lax
-    paths:
-      - third_party$
-      - builtin$
-      - examples$
+run:
+  timeout: 10m