Docs: Integration test conventions and fixtures

[mjkatgithub]

Planning

• Milestone: Phase 4
• Priority: P2
• Size: M
• Estimate: 3 SP
• Epic: Epic: Expand integration test suite #128

Goal

Document when to write integration vs unit vs E2E tests and add shared
fixtures so new issues can fill the integration checklist meaningfully.

Scope

• Create docs/testing-integration.md (or section in README)
• Optional tests/integration/support/ for shared Matrix room fixtures
• Update feature issue template examples if needed
• Cross-link from README "Test layers"

Branch

feature/integration-test-conventions

Acceptance Criteria

• Doc lists decision tree: unit / integration / E2E
• At least one copy-paste integration spec skeleton linked
• README points to the doc

Test Checklist

• Solo walkthrough: pick a recent feature, classify tests per doc

Notes

Docs: primary deliverable for this issue.