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

.gitignore

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