docs: add docs folder (architecture, developer, usage); update README; wire probe TTL cache + concurrency notes into docs

docs/developer/setup.md

@@ -0,0 +1,26 @@
+# Developer Setup
+
+## Prerequisites
+- Rust toolchain (stable)
+- Node.js (LTS) for control/ui
+- Docker (optional) for local stack
+
+## Build
+```bash
+cargo build
+cd control/ui && npm ci && npm run build
+```
+
+## Workspace Verification
+```bash
+cargo fmt --check
+cargo clippy --workspace --all-targets -- -D warnings
+cargo test --workspace
+cd control/ui && npm ci && npm run lint && npm run typecheck && npm run test && npm run build
+```
+
+## Environment
+- Gateway: routing config using file or KV
+- Projection: PROJECTION_GRPC_ADDR
+- Runner: RUNNER_GRPC_ADDR
+- NATS: URLs via service-specific settings