CRY-56: docs(linear): add Ash domain ERD and AGENTS.md maintenance contract (#142)

## 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

- [x] `mix compile --warnings-as-errors` passes (no Elixir changes)
- [x] `mix test` — 12 tests, all pass (no code changes)
- [x] Document is AsciiDoc under `documents/`, accurately reflects the
code
- [x] Every current resource, action, and code interface is represented
- [x] All associations labeled by implementation type
(GraphQL/nested-data vs Ash DSL)
- [x] Diagram has a complete plain-text table equivalent
- [x] `AGENTS.md` maintenance contract is specific and actionable
- [x] `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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: bougyman

AGENTS.md

@@ -93,6 +93,15 @@ Update this section when adding a new top-level directory, a new major module
 or a new phase plan document. File-level changes inside existing directories do
 not need a structural-doc update.
 
+## Domain documentation
+
+- Ash domain ERD: documents/ash-domain-erd.adoc — canonical inventory of
+  the `LinearCli.Linear` domain: resources, actions, code interfaces,
+  GraphQL associations, and shared helpers. **Must be updated in the same
+  change** whenever an Ash resource, action, code interface, association
+  attribute, or shared helper is added, removed, renamed, or materially
+  changed.
+
 ## Standards
 
 - Conventional Commits: app/usage-rules.md — enforced by the `commit-msg`