isolint: add skip-fields config for domain collisions

[yanyi-wego]

Summary

• Adds a skip-fields setting to isolint that suppresses ISO-code diagnostics on string literals assigned to named struct fields.
• Motivating false positive: a.CardSchemes = pq.StringArray{"MC"} in payments flagged as Monaco even though "MC" is the MasterCard scheme abbreviation. "AX" (American Express vs Åland Islands) is the same shape.
• Configurable via .golangci.yml (linters.settings.custom.isolint.settings.skip-fields) when used as a module plugin, and via -skip-fields=... on the standalone CLI.

Approach

The check is field-name syntactic, not type-based. Two AST shapes are recognised in isAssignToSkipField:

a.CardSchemes = pq.StringArray{"MC"}                          // AssignStmt + SelectorExpr LHS
_ = entity.CardAvailability{CardSchemes: pq.StringArray{"MC"}} // KeyValueExpr inside CompositeLit

The walk stops at function boundaries so a literal beyond a FuncLit/FuncDecl is no longer treated as part of the original assignment.

Two alternatives were considered and rejected — see docs/decisions.md:

• Type-based match (e.g. "skip everything assigned to a pq.StringArray") would require pass.TypesInfo and force LoadModeTypesInfo on the whole golangci-lint batch — a project-wide cost called out in docs/decisions.md "LoadModeSyntax". Also coarser — would suppress legitimate pq.StringArray of country codes too.
• Flat value allowlist (allow-values: [MC]) would suppress the same literal everywhere, including in genuine country contexts.

Settings flows through both entry points: the module plugin decodes via register.DecodeSettings[Settings] in plugin.go, and the standalone CLI registers a -skip-fields flag on the analyzer's FlagSet in NewAnalyzer, seeded from Settings.SkipFields so plugin and analystest callers (which don't parse flags) get the configured behaviour from the seeded default. The package-level Analyzer = NewAnalyzer(Settings{}) is preserved for backward-compatibility with existing tests and the standalone CLI's zero-config default.

Test plan

• TestAnalyzerWithSkipFields runs analystest.Run against a new testdata/skipfields/ package with Settings{SkipFields: ["CardSchemes"]}.
  • skip_fields.go — Pattern A and Pattern B fixtures with no // want annotations (asserts zero diagnostics under the configured skip).
  • still_flags.go — same package, same values, but assigned to IssuerCountries or as bare literals; asserts the skip is field-scoped, not global.
• TestSkipFieldsFlag and TestSkipFieldsFlagSeededFromSettings in flags_test.go cover the flag.Value wiring on the analyzer's FlagSet.
• TestRunWithNilPkg updated for the new run(pass, skipFields) signature; original golangci-lint nil-pass.Pkg guard still verified.
• Existing TestAnalyzer, TestAnalyzerWithFixes continue to pass against ./example (unchanged, asserts zero regressions on the original surface).
• CLI sanity check: isolint -skip-fields=CardSchemes against testdata/skipfields/ reports 5 diagnostics; without the flag, 10. The 5 suppressed are the configured CardSchemes assignments.