Fix golangci-lint CI: upgrade to v2 + resolve all findings

[YASSERRMD]

Summary

The lint job failed with "the Go language version (go1.23) used to build golangci-lint is lower than the targeted Go version (1.25.0)". golangci-lint v1.61.0 cannot analyze a module that targets Go 1.25.

• Upgrade golangci-lint to v2.12.2 via golangci-lint-action@v8, and migrate .golangci.yml to the v2 config format.
• With the linter now actually running (it never could before), resolve all 53 findings it surfaced across phase 36-55 and pre-existing code:
  • removed/derived unused declarations (canonical, listing, validateURL, ensureFmt; exported RunParallel)
  • checked previously-ignored session.Wait() returns in the 12 MCP server mains
  • renamed Run_ field to RunFn (var-naming) and updated call sites
  • renamed unused params to _ across several funcs
  • replaced hand-rolled string/int helpers with stdlib (strings.ReplaceAll, fmt.Sprintf)
  • stored pooled buffers by pointer (SA6002); escaped the zero-width space literal (ST1018); merged the connected-components closure decl
  • documented gosec excludes for genuine false-positives (G101 DSN default, G118 detached verification ctx, G404 non-crypto sampling, G705 html/template auto-escape, G124 locale cookie on plain-HTTP .onion); disabled noisy govet shadow and revive redefines-builtin-id

Test plan

• golangci-lint run --timeout=5m -> 0 issues
• gofmt -l . -> clean
• go build ./... and go test ./... -> all pass