chore: Add agentic coding tooling

.crosslink/rules/typescript.md

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+### TypeScript Best Practices
+
+#### Warnings Are Errors - ABSOLUTE RULE
+- **ALL warnings must be fixed, NEVER silenced**
+- No `// @ts-ignore`, `// @ts-expect-error`, or `eslint-disable` without explicit justification
+- No `any` type - use `unknown` and narrow with type guards
+- Fix the root cause, don't suppress the symptom
+
+```typescript
+// FORBIDDEN: Silencing warnings
+// @ts-ignore
+// eslint-disable-next-line
+const data: any = response;
+
+// REQUIRED: Fix the actual issue
+const data: unknown = response;
+if (isValidUser(data)) {
+    console.log(data.name);  // Type narrowed safely
+}
+```
+
+#### Code Style
+- Use strict mode (`"strict": true` in tsconfig.json)
+- Prefer `interface` over `type` for object shapes
+- Use `const` by default, `let` when needed, never `var`
+- Enable `noImplicitAny`, `strictNullChecks`, `noUnusedLocals`, `noUnusedParameters`
+
+#### Type Safety
+```typescript
+// GOOD: Explicit types and null handling
+function getUser(id: string): User | undefined {
+    return users.get(id);
+}
+
+const user = getUser(id);
+if (user) {
+    console.log(user.name);  // TypeScript knows user is defined
+}
+
+// BAD: Type assertions to bypass safety
+const user = getUser(id) as User;  // Dangerous if undefined
+```
+
+#### Error Handling
+- Use try/catch for async operations
+- Define custom error types for domain errors
+- Never swallow errors silently
+- Log errors with context before re-throwing
+
+#### Security - CRITICAL
+- **Validate ALL user input** at API boundaries (use zod, yup, or io-ts)
+- **Sanitize output** - use DOMPurify for HTML, escape for SQL
+- **Never use**: `eval()`, `Function()`, `innerHTML` with user data
+- **Use parameterized queries** - never string concatenation for SQL
+- **Set security headers**: CSP, X-Content-Type-Options, X-Frame-Options
+- **Avoid prototype pollution** - validate object keys from user input
+
+```typescript
+// GOOD: Input validation with zod
+import { z } from 'zod';
+const UserInput = z.object({
+    email: z.string().email(),
+    age: z.number().min(0).max(150),
+});
+const validated = UserInput.parse(untrustedInput);
+
+// BAD: Trust user input
+const { email, age } = req.body;  // No validation
+```
+
+#### Dependency Security - MANDATORY
+- Run `npm audit` before every commit - **zero vulnerabilities allowed**
+- Run `npm audit fix` to patch, `npm audit fix --force` only with review
+- Use `npm outdated` weekly to check for updates
+- Pin exact versions in production (`"lodash": "4.17.21"` not `"^4.17.21"`)
+- Review changelogs before major version upgrades
+- Remove unused dependencies (`npx depcheck`)
+
+```bash
+# Required checks before commit
+npm audit              # Must pass with 0 vulnerabilities
+npm outdated           # Review and update regularly
+npx depcheck           # Remove unused deps
+```
+
+#### Forbidden Patterns
+| Pattern | Why | Fix |
+|---------|-----|-----|
+| `any` | Disables type checking | Use `unknown` + type guards |
+| `@ts-ignore` | Hides real errors | Fix the type error |
+| `eslint-disable` | Hides code issues | Fix the lint error |
+| `eval()` | Code injection risk | Use safe alternatives |
+| `innerHTML = userInput` | XSS vulnerability | Use `textContent` or sanitize |