Fix nil pointer map values not being marshaled (#1025)

When marshaling a map with nil pointer values, the keys were being
silently dropped, breaking round-trip fidelity. For example:

    map[string]*struct{}{"foo": nil}

Would produce an empty TOML document instead of "[foo]".

This change converts nil pointer values in maps to their zero values
(consistent with how nil pointers in slices are handled), allowing the
keys to be preserved as empty tables.

Nil interface values (map[string]any{"foo": nil}) are still skipped
since there's no type information to derive a zero value.

Fixes #975

Also, pin golangci-lint version to v2.8.0 in CI and document in AGENTS.md

- Explicitly set golangci-lint version in lint.yml to ensure consistent
  behavior across CI runs
- Update AGENTS.md with instructions to use the same linter version locally

---------

Co-authored-by: Claude <noreply@anthropic.com>

AGENTS.md

@@ -40,6 +40,13 @@ go-toml is a TOML library for Go. The goal is to provide an easy-to-use and effi
 
 - Follow existing code format and structure
 - Code must pass `go fmt`
+- Code must pass linting with the same golangci-lint version as CI (see version in `.github/workflows/lint.yml`):
+  ```bash
+  # Install specific version (check lint.yml for current version)
+  curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/HEAD/install.sh | sh -s -- -b $(go env GOPATH)/bin <version>
+  # Run linter
+  golangci-lint run ./...
+  ```
 
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