Improve env binding, type support, and CI/docs alignment

[spiegel-im-spiegel]

Summary

This PR consolidates repository maintenance work and functional enhancements in struct2pflag.

• Add environment-variable binding API:
  • BindEnv(v any, opts ...EnvOpt) error
  • options: WithEnvPrefix, WithEnvTag, WithStrict
  • sentinel errors compatible with errors.Is
• Extend Bind to support additional types already supported by pflag:
  • float32, float64, time.Duration
• Add/expand tests for env binding and new bindable types.
• Add comments clarifying why time.Duration is handled in a dedicated path.
• Align workflows and docs:
  • keep ci.yml + codeql.yml
  • remove legacy lint.yml
  • update README badges/sections and usage notes
  • add English env-bind specification doc
• Update example behavior/documentation around JSON parsing warning handling.

Validation

• go test -shuffle on ./...
• golangci-lint-v2 run --enable gosec --timeout 3m0s ./...
• task test

All checks pass locally.

Notes

• Scope includes both maintenance alignment and feature additions that were developed incrementally on devel.

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