GH-85: fix(linear): follow GraphQL cursors past first:50 + add updatedAt delta filter

[alekspetrov]

Summary

Automated PR created by Pilot for task GH-85.

Closes #85

Changes

GitHub Issue GH-85: fix(linear): follow GraphQL cursors past first:50 + add updatedAt delta filter

Context

SaaS pre-work S0.4. The Linear client lists issues with first: 50 and does not follow GraphQL page cursors — any project with more than ~50 issues is silently truncated. It also lacks an updatedAt delta filter, forcing full refetches. Both block the SaaS board-sync ingest (delta polling with watermark cursors) and are latent bugs for today's poller on large Linear projects.

Implementation

In sdk/integrations/linear:

1. Cursor-follow all issue list queries: loop pageInfo { hasNextPage endCursor } until exhausted (page size 50–100), preserving current return shape.
2. Add an updatedAt-filtered list method (GraphQL issues(filter: { updatedAt: { gt: $since } }, orderBy: updatedAt)) with the same cursor-following, for delta polling.
3. Keep existing exported method signatures backward-compatible; new capability may be a new method rather than changing an existing signature.

Acceptance

• List queries return >50 issues completely (test with mocked paged GraphQL responses: 3 pages, assert union)
• Delta method returns only issues with updatedAt > since, cursor-followed
• No behavioral change for projects under one page
• Existing linear connector tests green

Refs

• Verified defect: qf-studio/pilot .agent/system/saas-asset-research.md § studio-sdk (Linear first:50 no-cursor bug)
• Consumer design: .agent/system/saas-kanban-sync-design.md §2 (per-provider delta mechanics table)