Add Outline API client library foundation

Implements a first vertical slice of the Outline RPC API (auth, documents,
collections, users) covering the structural patterns used across the whole
API: single object, paginated list, tree, create/update, delete, search.
Chosen as the basis for future CLI and GUI clients built on top of this crate.

- Handwritten client (not codegen) against the vendored OpenAPI spec, since
  Outline's API is uniformly POST /api/<resource>.<action> with JSON bodies
  and inline/anonymous schemas that generators handle poorly
- Async (reqwest + tokio) Client, cheaply cloneable, no &mut self methods
- thiserror-based Error with ErrorKind classification and boxed API error
  context; forward-compatible models (unknown fields ignored, unknown
  string-enum values preserved via a catch-all variant) since the API is
  unversioned and self-hosted instances vary in age
- Pagination via Page<T>/Paginator with next_page/collect_all/into_stream
- wiremock-based test suite plus a spec-coverage test guarding against typos
  in RPC method names

tests/live.rs

@@ -0,0 +1,29 @@
+//! Smoke tests against the real Outline API.
+//!
+//! Ignored by default. Run with:
+//! `OUTLINE_API_KEY=ol_api_... cargo test -- --ignored`
+
+use outline::Client;
+
+#[tokio::test]
+#[ignore]
+async fn auth_info_and_collections_list_succeed() {
+    let client =
+        Client::from_env().expect("OUTLINE_API_KEY (and optionally OUTLINE_URL) must be set");
+
+    let auth = client
+        .auth()
+        .info()
+        .await
+        .expect("auth.info should succeed with valid credentials");
+    println!("Signed in as {} ({})", auth.user.name, auth.team.name);
+
+    let page = client
+        .collections()
+        .list()
+        .limit(5)
+        .send()
+        .await
+        .expect("collections.list should succeed");
+    println!("Fetched {} collection(s)", page.items.len());
+}