feat(content): introduce typed content envelope

[yordis]

• Domain messages that need an open-schema field today either reinvent a content_type + bytes pair, reach for google.protobuf.Any (which forces consumers to know the inner message), or smuggle JSON through string. A stable, named envelope keeps the contract explicit and avoids divergence across packages.
• Namespaced as trogon.content.v1alpha1.Content so the package itself conveys context; the message is the noun that the field content_type classifies. Keeps the single-noun-package convention used by trogon.actor.Actor and trogon.uuid.Uuid.
• Deliberately minimal — just content_type and data. No extensions/Any side channel until a concrete need shows up, to avoid an ungoverned escape hatch (what belongs there vs. headers vs. domain fields vs. event metadata is unanswered today).

[cursor]

PR Summary

Low Risk
Additive proto-only change with no runtime or consumer wiring in this diff.

Overview
Introduces a shared trogon.content.v1alpha1.Content protobuf envelope so APIs can carry open or negotiated payloads without each package inventing its own content_type + bytes pair, abusing google.protobuf.Any, or overloading string for JSON.

The message is intentionally minimal: content_type (IANA media type per RFC 6838) and opaque data. Documentation steers callers toward explicit domain messages when the schema is fixed, and toward Content only when encoding is agreed out of band. The package follows the same v1alpha1 + elixirpb module-prefix pattern as other Trogon shared types (e.g. actor).

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• proto/trogon/content/v1alpha1/content.proto

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• Create PR with unit tests
• Commit unit tests in branch yordis/add-content

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[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	May 27, 2026, 10:54 PM