feat: add project_label stream

[grawinkel]

Summary

Adds support for syncing project labels from Linear, following the existing pattern used by issue_label and other streams.

Linear's export API supports the projectlabel entity type, but the connector currently doesn't expose it as a stream. This PR adds it.

Changes

File	Change
src/client/types.ts	Add ProjectLabel interface
src/client/LinearClient.ts	Add projectLabels() method + "projectlabel" entity type
src/streams/projectLabel.ts	New stream class (same pattern as issueLabel.ts)
resources/schemas/projectLabel.json	JSON schema for ProjectLabel
src/index.ts	Register ProjectLabel stream

Schema

The ProjectLabel model mirrors IssueLabel but without teamId and organizationId:

interface ProjectLabel {
  id?: string;
  createdAt?: string;
  updatedAt?: string;
  archivedAt?: null | string;
  name?: string;
  description?: null | string;
  color?: string;
  creatorId?: null | string;
}

Context

We use this connector to sync Linear data into BigQuery via Airbyte. We need project_label to join project labels with projects for reporting and analytics. The Linear GraphQL API exposes projectLabels as a top-level query, and the export API should support projectlabel as an entity type.