docs(architecture): ADR for plugins and extensions taxonomy

[seanspeaks]

Summary

Adds docs/architecture/ADR-EXTENSIONS.md — a proposed three-tier taxonomy for what's pluggable in Frigg:

• Core Plugins — required infrastructure with defaults (provider, queue, encryption, DB). Already covered by ADR-MULTI-PROVIDER-SUPPORT; this ADR names the category for contrast.
• Application Extensions — optional, declarative additions to the core app, listed on appDefinition.extensions. Grounded in the packages/core/extensions/ prototype on claude/frigg-netlify-exploration-aY2Bh and the reference @friggframework/extension-db-credentials package.
• Integration Extensions — off-the-shelf handler bundles (webhooks, sync routines) shipped with API modules, bound into an integration's Definition.extensions. Grounded in the this.extensions prototypes in lefthookhq/frigg-2.0-prototyping, stack-global--frigg-2.0, and vartopia--frigg-2.0.

What the ADR resolves

• Vocabulary collision. "Plugin" and "extension" are currently used for at least three different things across the codebase and prototypes.
• Shape mismatch. The Tier 2 loader expects extensions: [...] (array); the Tier 3 prototypes use extensions: { name: {...} } (object). The ADR keeps both — they model different problems — and explains why.
• this.extensions vs static Definition.extensions. Recommends moving Tier 3 wiring out of the constructor and onto the static Definition, with string-name handler binding done by the framework at instantiation.

Status

• Status: Proposed — open for review and revision before acceptance.
• No code changes. Pure design doc.
• Implementation phases sketched at the end of the ADR (Tier 2 land → Tier 3 formalize → docs → deprecate old shape).

Open questions in the ADR

1. Handler-binding-by-string trade-offs vs. a factory form
2. Whether Tier 3 should be able to contribute Prisma models (currently a Tier 2 capability)
3. Versioning / compatibility metadata on extensions

Test plan

• Confirm the three-tier model maps cleanly to the existing Multi-Provider ADR (Tier 1)
• Confirm Tier 2 description matches the netlify-exploration prototype's intent
• Confirm Tier 3 description matches the LeftHook downstream prototypes' intent
• Resolve open questions before marking Accepted

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[claude]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud