Add a complete Copilot configuration for regorus with layered architecture:
instructions, knowledge files, skills, role-based agents, and CI validation.

## Instructions (auto-loaded)
- copilot-instructions.md: lean 5KB orientation with knowledge file references
- copilot-code-review-instructions.md: "think freely" review guide with
  severity categories and domain-specific context (Undefined, FFI, telemetry)

## Knowledge Files (20 files, ~70KB)
Deep institutional knowledge organized by subsystem:
- Core: value-semantics, engine-api, error-handling-migration
- Execution: interpreter-architecture, rvm-architecture, compilation-pipeline
- Rego: rego-semantics, rego-compiler, builtin-system
- Azure: azure-policy-language, azure-policy-aliases, azure-rbac-language
- Safety: policy-evaluation-security, ffi-boundary, feature-composition
- Diagnostics: telemetry-and-diagnostics, causality-and-partial-eval
- Extensibility: language-extension-guide, tooling-architecture, time-builtins-compat

## Agents (16 role-based personas)
Each agent brings a distinct thinking mode for review and planning:
- Core engineering: red-teamer, semantics-expert, architect, performance-engineer
- Quality: test-engineer, verification-engineer, security-auditor
- Operations: reliability-engineer, support-engineer, ci-engineer
- Evolution: refactorer, api-steward
- Product: program-manager, demo-engineer, dx-engineer
- Leadership: tech-lead (with 9 constitutional rules and cross-agent synthesis)

## Skills (6 task workflows)
- thorough-review: multi-agent parallel review with cross-agent context protocol
- design-alternatives: multi-approach evaluation against 9 dimensions
- add-builtin, opa-conformance, security-review, verification

## CI & Infrastructure
- copilot-setup-steps.yml: Rust 1.92.0 + clippy + rustfmt + cargo cache
- copilot-config-validation.yml: YAML syntax, reference checking, staleness detection

## Architecture Documentation
- docs/copilot-architecture.md: how all layers connect, extension guide

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>