docs: engineering knowledge base for new hires

Complete docs/engineering/ suite — 12 documents covering company overview,
system architecture, tech stack ADRs, codebase structure, service deep dives,
annotated code walkthroughs, dev setup, engineering workflow, testing strategy,
deployment/ops, SDK guide, and README index. All content verified against
source files. All 82 tasks in openspec/changes/engineering-docs/tasks.md
marked complete.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/engineering/README.md

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+# SentryAgent.ai — Engineering Knowledge Base
+
+> Internal reference for engineers contributing to AgentIdP. Read in order if you're new. Jump to the relevant document if you know what you need.
+
+---
+
+## Reading Order (New Engineers Start Here)
+
+| # | Document | What you'll learn | Time |
+|---|---------|------------------|------|
+| 1 | [Company and Product Overview](01-overview.md) | What SentryAgent.ai builds, why it exists, the product feature set, Phase roadmap | 15 min |
+| 2 | [System Architecture](02-architecture.md) | Component diagram, HTTP request lifecycle, OAuth 2.0 data flow, multi-region topology | 20 min |
+| 3 | [Technology Stack and ADRs](03-tech-stack.md) | Why each technology was chosen — rationale and alternatives considered | 20 min |
+| 4 | [Codebase Structure](04-codebase-structure.md) | Directory map, where to add new code, DRY enforcement rules | 15 min |
+| 5 | [Service Deep Dives](05-services.md) | All 8 services/components — purpose, interface, schema, error types | 30 min |
+| 6 | [Annotated Code Walkthroughs](06-walkthroughs.md) | Step-by-step traces of token issuance, agent registration, credential rotation | 30 min |
+| 7 | [Development Environment Setup](07-dev-setup.md) | Clone to running local stack — under 30 minutes | 30 min |
+| 8 | [Engineering Workflow](08-workflow.md) | OpenSpec spec-first workflow, branching, PR checklist, commit conventions | 20 min |
+| 9 | [Testing Strategy](09-testing.md) | Unit vs integration, coverage gates, how to write tests, OWASP reference | 20 min |
+| 10 | [Deployment and Operations](10-deployment.md) | Docker, Terraform, Prometheus/Grafana, operational runbook | 20 min |
+| 11 | [SDK Integration Guide](11-sdk-guide.md) | All 4 SDKs — installation, examples, contribution guide | 20 min |
+
+**Total estimated reading time for new engineers: ~3.5 hours**
+
+---
+
+## Quick Reference
+
+| I need to... | Go to |
+|-------------|-------|
+| Understand the codebase layout | [04-codebase-structure.md](04-codebase-structure.md) |
+| Run the project locally | [07-dev-setup.md](07-dev-setup.md) |
+| Understand how token issuance works end-to-end | [06-walkthroughs.md](06-walkthroughs.md) |
+| Add a new API endpoint | [08-workflow.md](08-workflow.md) + [04-codebase-structure.md](04-codebase-structure.md) |
+| Write tests | [09-testing.md](09-testing.md) |
+| Deploy to production | [10-deployment.md](10-deployment.md) |
+| Integrate with the SDK | [11-sdk-guide.md](11-sdk-guide.md) |
+| Understand why a technology was chosen | [03-tech-stack.md](03-tech-stack.md) |
+
+---
+
+## Document Conventions
+
+- **File paths** are always relative to the project root unless otherwise noted.
+- **Line numbers** in [06-walkthroughs.md](06-walkthroughs.md) were verified against commit `1f95cfe`.
+- **Code examples** are complete and runnable — no ellipses, no placeholders.
+- **ADR** stands for Architecture Decision Record — a short document recording a technology choice.
+
+---
+
+## Related Documentation
+
+- `docs/developers/` — End-user API reference (for agents calling the AgentIdP API)
+- `docs/devops/` — Operator runbooks and environment variable reference
+- `docs/agntcy/` — AGNTCY alignment documentation
+- `openspec/` — OpenSpec change management (proposals, designs, specs, tasks, archives)