Setup repo: migrate architecture specs, code stubs, and tasks from alkhub_ts

Copy architecture docs, ADRs, storage domain specs, research, reviews,
and 56 storage architecture tasks from the alkhub_ts monorepo. Adapt for
standalone @alkdev/hub repo structure (src/ not packages/hub/).

Sanitize all sensitive information:
- Replace private IPs (10.0.0.1) with localhost defaults
- Remove internal server hostnames (dev1, ns528096)
- Replace /workspace/ private paths with npm package references
- Remove hardcoded credentials from examples
- Rewrite infrastructure.md without private network details

Add Deno project scaffolding: deno.json (pinned deps), .gitignore,
AGENTS.md, entry point. Migrate existing code stubs (crypto, config
types, logger) with updated import paths.

tasks/architecture/storage/document-mappings-valid-shapes.md

@@ -0,0 +1,38 @@
+---
+id: document-mappings-valid-shapes
+name: Document Valid Column Combinations for Mappings Table
+status: completed
+depends_on: []
+scope: narrow
+risk: medium
+impact: component
+level: implementation
+---
+
+## Description
+
+W20: The `mappings` table stores three conceptually different relationships in one table: session→spoke, session→parent session, session→task. All nullable FKs allow any combination, including invalid ones. The table name `mappings` doesn't convey what's mapped.
+
+Document the valid column combinations (polymorphic association shapes): `sessionId` always NOT NULL; `taskId` only for task-scoped mappings; `parentSessionId` only for coordinator children. This makes it a polymorphic association table with documented shapes.
+
+## Acceptance Criteria
+
+- [ ] `coordination.md` documents valid column combinations (mapping shapes)
+- [ ] Shape 1: Session→Spoke (`sessionId` + `spokeId`, no `taskId`/`parentSessionId`)
+- [ ] Shape 2: Session→Parent (`sessionId` + `parentSessionId`, no `taskId`)
+- [ ] Shape 3: Session→Task (`sessionId` + `taskId`, no `parentSessionId`)
+- [ ] Invalid combinations documented (e.g., `taskId` + `parentSessionId` on same row)
+- [ ] `sessionId` noted as always NOT NULL for any mapping
+
+## References
+
+- docs/reviews/storage-architecture-review-2026-04-21.md#W20
+- docs/architecture/storage/coordination.md:10-27
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion