CRY-56: docs(linear): add Ash domain ERD and AGENTS.md maintenance contract

[bougyman]

Summary

• Creates documents/ash-domain-erd.adoc as the canonical, current inventory of the LinearCli.Linear Ash domain
• Covers all 8 resources, 20 actions, 17 domain code interfaces, GraphQL/nested-data associations, and shared helpers
• Adds a ## Domain documentation section to AGENTS.md with an explicit same-change maintenance contract

What the ERD document contains

• Architecture overview: explains no local data layer — the Linear GraphQL API is the sole backing store; all actions are manual implementations calling LinearCli.Api directly
• Resource inventory table: all 8 resources with primary key and public attributes
• Association table: every source→target association with cardinality, attribute name, and implementation type (all labeled "GraphQL/nested-data" — zero Ash DSL relationships exist)
• ERD diagram (Mermaid erDiagram block) with all associations labeled [nested]
• Action/interface matrix: maps every domain code interface to its action, action type, manual module, and GraphQL operation (20 rows covering all 17 defined code interfaces plus any action-only entries)
• Shared helpers section: Issue.Update (shared issueUpdate mutation runner for 4 update actions) and Paginate (cursor-based pagination used by 3 read actions)
• Maintenance contract section (also mirrored into AGENTS.md)

Test plan

• mix compile --warnings-as-errors passes (no Elixir changes)
• mix test — 12 tests, all pass (no code changes)
• Document is AsciiDoc under documents/, accurately reflects the code
• Every current resource, action, and code interface is represented
• All associations labeled by implementation type (GraphQL/nested-data vs Ash DSL)
• Diagram has a complete plain-text table equivalent
• AGENTS.md maintenance contract is specific and actionable
• documents/project-structure.adoc cross-link deferred (CRY-55 not yet merged)

Closes https://linear.app/cryptokairos/issue/CRY-56/document-and-maintain-the-ash-domain-erd

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[bougyman]

Rework — run 2

Addressed review feedback: "This is built as a table. Can we use mermaid to make a better looking ERD?"

The document already contained a Mermaid erDiagram block, but it appeared after the association table and showed only bare entity names connected by edges. Changes made:

• Moved the ERD diagram to immediately follow the architecture overview — it is now the first thing you see after the intro
• Expanded the Mermaid erDiagram to include full entity attribute boxes for all 8 resources, using the target resource name as the attribute type for nested-struct fields (e.g. User assignee, Team[] teams, WorkflowState state) — this makes it look like a proper ERD rather than just a relationship graph
• Relabeled the resource and association tables as "text reference" (accessible equivalents), clarifying their role as the plain-text alternative to the diagram rather than the primary representation

No changes to the action/interface matrix, shared helpers, or maintenance contract sections.