feat(gitlab): add GraphQL client with adaptive pagination and work item status types

Introduce a reusable GraphQL client (`src/gitlab/graphql.rs`) that handles
GitLab's GraphQL API with full error handling for auth failures, rate
limiting, and partial errors. Key capabilities:

- Adaptive page sizing (100 → 50 → 25 → 10) to handle GitLab GraphQL
  complexity limits without hardcoding a single safe page size
- Paginated issue status fetching via the workItems GraphQL query
- Graceful detection of unsupported instances (missing GraphQL endpoint
  or forbidden auth) so ingestion continues without status data
- Retry-After header parsing via the `httpdate` crate for rate limit
  compliance

Also adds `WorkItemStatus` type to `gitlab::types` with name, category,
color, and icon_name fields (all optional except name) with comprehensive
deserialization tests covering all system statuses (TO_DO, IN_PROGRESS,
DONE, CANCELED) and edge cases (null category, unknown future values).

The `GitLabClient` gains a `graphql_client()` factory method for
ergonomic access from the ingestion pipeline.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -1118,6 +1118,7 @@ dependencies = [
  "dirs",
  "flate2",
  "futures",
+ "httpdate",
  "indicatif",
  "libc",
  "open",