feat(adr): ADR 0027 — Architecture as Code: adopt CALM (FINOS)

[MLKiiwy]

Summary

Write ADR 0027 deciding to adopt CALM (Common Architecture Language Model, FINOS open standard) as the Architecture as Code language for Alistigo. Document the rationale for choosing CALM over C4/Structurizr/LikeC4/ArchiMate.

Acceptance criteria

• ADR 0027 written at docs/adrs/0027-architecture-as-code-calm.md
• ADR covers: context (problem with static Markdown), alternatives considered, decision rationale, consequences
• ADR index updated (docs/adrs/README.md)

Context

Part of the Architecture as Code epic. The ADR captures: why AasC now (AI drift risk), why CALM (JSON-first, open standard, FINOS, MCP tooling, visualisation), what alternatives were rejected and why (C4, Structurizr DSL, LikeC4, ArchiMate, custom JSON).

References: https://calm.finos.org, https://www.catio.tech/blog/architecture-as-code

Epic: architecture-as-code