feat(template): derive ErrorTemplate from proto Template option - #20

PR Summary

Medium Risk
New public API controls error shape and metadata visibility from proto annotations; misconfiguration could leak or omit fields, though behavior is covered by tests and existing templates are unchanged unless adopters use the new helpers.

Overview
Adds proto-driven error templates so Go ErrorTemplate values can be built from trogon.error.v1alpha1 message and field options instead of duplicating domain, reason, code, visibility, help, and metadata in code.

NewErrorTemplateFromProto reads the message template option once (and caches field specs); FromProto turns a populated proto into a TrogonError, mapping annotated fields to metadata (fixed value, default_value fallbacks, runtime overrides, and caller ErrorOptions winning last). ErrorTemplate.NewError now also applies template-level metadata baked at construction time.

Tooling: buf.gen.yaml, internal test protos + generated Go under internal/testdata/gen, a CI Generated Code Up-To-Date job (buf generate + clean git diff), .gitattributes for generated files, Go 1.26, and a direct dependency on github.com/TrogonStack/trogonproto.

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