docs: add core concepts (sense, signal, reflex, workflow) to agent rules

.github/copilot-instructions.md

@@ -1,5 +1,35 @@
 # Nerve Coding Conventions
 
+## Core Concepts
+
+```
+External World → Sense → Signal → Reflex → Workflow → Log
+                  ↑                                     ↑
+              "what to observe"                  "what to do"
+```
+
+**Nerve** is a lightweight observation engine daemon for autonomous agents. It continuously observes external state, reacts to changes via declarative rules, and orchestrates multi-step workflows.
+
+### Key Terms
+
+| Concept | What it is |
+|---------|-----------|
+| **Sense** | A `compute()` function that samples or derives data. Returns `T \| null` — non-null emits a Signal, null is silent. Each Sense has its own SQLite database. |
+| **Signal** | A notification emitted when a Sense returns non-null. Pure fact, no intent. Distributed via an in-memory Signal Bus. Not persisted. |
+| **Reflex** | A declarative trigger (YAML) connecting Senses to actions. Trigger types: `interval` (periodic), `on` (react to Signals). Action types: trigger a Sense, or start a Workflow. |
+| **Workflow** | A stateful multi-step execution. Contains **Roles** (actors with side effects) and a **Moderator** (pure router). Each instance is a **Thread** with a unique `runId`. |
+| **Log** | Immutable audit trail. Records executions, state transitions, errors. **Cannot trigger Reflexes** — prevents feedback loops. |
+| **Engine** | The kernel orchestrating everything. Holds Signal Bus, Reflex Scheduler, Process Manager, Workflow Manager. Never loads user code directly — all user code runs in isolated Workers. |
+| **Daemon** | The `nerve-daemon` package — engine runtime. Runs as a background process. |
+
+### Architecture Rules
+
+- **Three orthogonal extension points**: Sense (what to compute), Reflex (when to compute), Workflow (what to do)
+- **Process isolation**: One worker per Sense group (long-lived), one per Workflow type (on-demand). Workers never talk to each other.
+- **Causality is one-directional**: External world → Sense → Signal → Reflex → Action + Log. Logs are the end of the chain.
+
+
+
 ## Language & Paradigm
 
 ### Functional-first