docs: vendor event-sourcing and Go architecture patterns; add engineering conventions

[malpou]

Replaces #2 (closed when its base branch was deleted on merging #1).

• docs/patterns/event-sourcing.md: the pattern only (six mechanisms, read path, catalog, memory bus, anti-patterns), vendored from an internal write-up with its migration plan and internal references removed, plus a note that omap-stack is Postgres only (the JetStream-specific parts do not apply). It is input, not a decision: which state is event-sourced is decided on the wayfinder map (ENG-322).
• docs/patterns/go-architecture-guidelines.md: a synthesis of Three Dots Labs guidance, with a neutral header.
• AGENTS.md: engineering conventions (in-tree Adapters picked from yaml, schema first, event-sourced by default per aggregate, Postgres as the only broker, testcontainers integration tests throughout).