feat(#756): add official Discord channel adapter

docs/reports/documentation/756-discord-plugin-checklist.md

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+# Documentation Completion Checklist — #756 Official Discord plugin
+
+## Required artifacts
+
+- [x] `docs/PRD.md` includes the #756 workstream, assumptions, and acceptance criteria.
+- [x] User workflow updated in `docs/tess/USER-GUIDE.md`.
+- [x] Administrator configuration and authorization policy updated in `docs/guides/admin-guide.md`.
+- [x] Developer/plugin authoring guidance updated in `docs/tess/PLUGIN-GUIDE.md`.
+- [x] Channel architecture updated in `docs/architecture/channel-protocol.md`.
+- [x] Package operations/development guide added at `plugins/discord/README.md`.
+- [x] `docs/SITEMAP.md` links the official channel plugin documentation.
+
+## API coverage
+
+- [x] No HTTP or WebSocket endpoint was added, removed, or changed.
+- [x] No OpenAPI update is needed.
+- [x] Shared TypeScript contracts are documented in architecture and plugin-authoring guides.
+- [x] Discord authentication, authorization, thread failure, and control-command behavior are documented.
+
+## Structural standards
+
+- [x] Working notes remain under `docs/scratchpads/`.
+- [x] Review and checklist artifacts remain under `docs/reports/`.
+- [x] No generated publishing output was added.
+- [x] Existing repository documentation structure was preserved; no unrelated root cleanup was attempted.
+
+## Review gate
+
+- [x] Independent documentation/contract review passes (shared-contract review plus final code review of the current documentation delta).
+- [x] Independent code review verifies documentation matches implementation (`docs/reports/code-review/756-code-review.md`: APPROVE).
+- [x] Independent security review verifies documented controls (`docs/reports/security/756-security-review.md`: APPROVE).
+
+## Publishing
+
+- [x] Canonical source remains in-repository.
+- [x] No external publishing action is in scope for this slice.