docs: add .github/copilot-instructions.md for GitHub Copilot

.github/copilot-instructions.md

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+# Nerve Coding Conventions
+
+## Language & Paradigm
+
+### Functional-first
+
+Use `function` + `type`, not `class` + `interface`.
+
+```typescript
+// ✅ Good
+type Signal = {
+  senseId: string;
+  value: unknown;
+  ts: number;
+};
+
+function createSignal(senseId: string, value: unknown): Signal {
+  return { senseId, value, ts: Date.now() };
+}
+
+// ❌ Bad — no class, no interface
+class Signal implements ISignal { ... }
+```
+
+### Rules
+
+| Rule | Description |
+|------|-------------|
+| `type` over `interface` | All type definitions use `type` |
+| `function` over `class` | Pure functions + closures, no class |
+| No `this` | Functions must not depend on `this` context |
+| No inheritance | No `extends`, `implements`, `abstract` |
+| Composition over inheritance | Use function composition |
+| Immutability first | Use `Readonly<T>`, `as const`, avoid mutation |
+| No optional properties | Use `T \| null` instead of `?:` — see below |
+
+### Exceptions
+
+Classes are allowed when:
+- Required by a third-party library (e.g. Drizzle's `sqliteTable`)
+- Error subclasses (`class NerveError extends Error`)
+
+### No Optional Properties
+
+Never use `?:`. All nullable fields must be explicit `T | null`.
+
+```typescript
+// ✅ Good
+type SenseConfig = {
+  group: string;
+  throttle: string | null;
+  timeout: string | null;
+};
+
+// ❌ Bad
+type SenseConfig = {
+  group: string;
+  throttle?: string;
+  timeout?: string;
+};
+```
+
+For mutually exclusive fields, use discriminated unions:
+
+```typescript
+// ✅ Good
+type ReflexConfig =
+  | { kind: "sense"; sense: string; interval: string | null; on: string[] | null }
+  | { kind: "workflow"; workflow: string; on: string[] | null };
+```
+
+## Modules & Exports
+
+- Always named exports, never default exports
+- One module = one responsibility, filename = purpose
+
+## Naming
+
+| Type | Style | Example |
+|------|-------|---------|
+| Files | kebab-case | `signal-bus.ts` |
+| Types | PascalCase | `SignalBus` |
+| Functions/variables | camelCase | `createSignalBus` |
+| Constants | UPPER_SNAKE | `MAX_RETRY_COUNT` |
+| Generics | Single letter or descriptive | `T`, `TValue` |
+
+## Error Handling
+
+- Use `Result` type for expected failures
+- `throw` only for unrecoverable bugs (programmer errors)
+- No try-catch for flow control
+
+```typescript
+type Result<T, E = Error> = { ok: true; value: T } | { ok: false; error: E };
+```
+
+## Async
+
+- Always `async/await`, never `.then()` chains
+
+## No Dynamic Import
+
+Do NOT use `await import()` in production code. Always use static top-level `import`.
+
+Exceptions (must include a comment):
+1. `sense-runtime.ts` — user module paths known only at runtime
+2. `workflow-worker.ts` — user module paths known only at runtime
+
+Test files (`__tests__/**`) are exempt.
+
+## Toolchain
+
+| Tool | Purpose |
+|------|---------|
+| **pnpm** | Package manager |
+| **TypeScript** | Type checking (strict mode) |
+| **Biome** | Lint + format (replaces ESLint + Prettier) |
+| **tsup** | Bundling |
+
+### Commands
+
+```bash
+pnpm run check      # biome check (lint + format)
+pnpm run format     # biome format --write
+pnpm run build      # full build
+pnpm test           # run tests
+```
+
+## Monorepo Structure
+
+```
+nerve/
+  packages/
+    core/           # @nerve/core — shared types and utils
+    cli/            # @nerve/cli — CLI entry point
+    daemon/         # @nerve/daemon — engine runtime
+  docs/             # RFCs, conventions
+```
+
+- `core` is the shared layer; `cli` and `daemon` both depend on it
+- `cli` and `daemon` must NOT depend on each other
+
+## Commit Convention
+
+```
+<type>(<scope>): <description>
+
+type: feat | fix | refactor | docs | chore | test
+scope: core | cli | daemon | rfc-001 | ...
+```