docs: add README and end-user guides for all modules

docs/validation.md

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+# Validation
+
+TypeBox-based schema validation helpers for operation inputs and outputs.
+
+## validateOrThrow
+
+Validates a value against a TypeBox schema. Throws on failure with formatted error messages:
+
+```ts
+import { validateOrThrow } from "@alkdev/operations";
+
+validateOrThrow(schema, input, "Input validation failed for task.create");
+```
+
+Used internally by `registry.execute()` to validate inputs before running handlers.
+
+## collectErrors
+
+Returns an array of errors without throwing:
+
+```ts
+import { collectErrors } from "@alkdev/operations";
+
+const errors = collectErrors(schema, value);
+// [{ path: "/title", message: "Expected string" }, ...]
+```
+
+Used internally by `registry.execute()` to warn on output validation failures.
+
+## assertIsSchema
+
+Validates that a value is a TypeBox schema. Throws if not:
+
+```ts
+import { assertIsSchema } from "@alkdev/operations";
+
+assertIsSchema(maybeSchema, "task.create input");
+```
+
+Useful when receiving schemas from external sources (JSON, OpenAPI) before passing them to the registry.
+
+## formatValueErrors
+
+Formats an iterable of `{ path, message }` errors into a human-readable string:
+
+```ts
+import { formatValueErrors } from "@alkdev/operations";
+
+const formatted = formatValueErrors(errors);
+// "  - /title: Expected string\n  - /priority: Expected union member"
+```
+
+Accepts an optional indent prefix (default `"  - "`).