chore: add golangci-lint, bump Go to 1.26, fix all lint issues (#133)

## Summary
- Add `.golangci.yml` with linter configuration matching the main gitea repo
- Add `lint`, `lint-fix`, `lint-go`, `lint-go-fix`, and `security-check` Makefile targets
- Add `tidy` Makefile target (extracts min Go version from `go.mod` for `-compat` flag)
- Bump minimum Go version to 1.26
- Update golangci-lint to v2.10.1
- Replace `golang/govulncheck-action` with `make security-check` in CI
- Add `make lint` step to CI
- Fix all lint issues across the codebase (formatting, `errors.New` vs `fmt.Errorf`, `any` vs `interface{}`, unused returns, stuttering names, Go 1.26 `new(expr)`, etc.)
- Remove unused `pkg/ptr` package (inlined by Go 1.26 `new(expr)`)
- Remove dead linter exclusions (staticcheck, gocritic, testifylint, dupl)

## Test plan
- [x] `make lint` passes
- [x] `go test ./...` passes
- [x] `make build` succeeds

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/133
Reviewed-by: techknowlogick <techknowlogick@noreply.gitea.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

.golangci.yml

@@ -0,0 +1,113 @@
+version: "2"
+output:
+  sort-order:
+    - file
+linters:
+  default: none
+  enable:
+    - bidichk
+    - bodyclose
+    - depguard
+    - errcheck
+    - forbidigo
+    - gocheckcompilerdirectives
+    - gocritic
+    - govet
+    - ineffassign
+    - mirror
+    - modernize
+    - nakedret
+    - nilnil
+    - nolintlint
+    - perfsprint
+    - revive
+    - staticcheck
+    - testifylint
+    - unconvert
+    - unparam
+    - unused
+    - usestdlibvars
+    - usetesting
+    - wastedassign
+  settings:
+    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:
+      concat-loop: false
+    govet:
+      enable:
+        - nilness
+        - unusedwrite
+  exclusions:
+    generated: lax
+    presets:
+      - comments
+      - common-false-positives
+      - legacy
+      - std-error-handling
+    rules:
+      - linters:
+          - errcheck
+          - staticcheck
+          - unparam
+        path: _test\.go
+issues:
+  max-issues-per-linter: 0
+  max-same-issues: 0
+formatters:
+  enable:
+    - gofmt
+    - gofumpt
+  settings:
+    gofumpt:
+      extra-rules: true
+  exclusions:
+    generated: lax
+run:
+  timeout: 10m