fix(linear): prevent duplicate Linear issues on batch retry

[cursor]

Summary

• add an idempotency pre-check in Tracker.BatchPush before batching unique-title creates
• if a marker already exists in Linear, reuse that issue URL instead of sending a new issueBatchCreate mutation
• add a regression test covering retry of an unlinked local issue to ensure the second push reuses the existing remote issue

Bug and impact

A successful batch create can leave a local issue unlinked if external_ref write-back fails locally. On the next sync, the same issue is treated as new and re-batch-created, producing duplicate Linear issues.

Root cause

Batch create path appended idempotency markers but did not check for existing markers before issuing new batch creates. Dedup checks existed for single-create but not for unique-title batch-create retries.

Fix

Perform FindIssueByDescriptionContains(marker) for each batch-create candidate:

• on hit: treat as created/deduped and reuse existing URL
• on miss: include in batch mutation as before
• on lookup error: record a per-issue error and continue safely

Validation

• go test -tags gms_pure_go ./internal/linear -run 'TestBatchPush_BatchCreateRetryUsesIdempotencySearch|TestBatchPush_AmbiguousBatchFailureSearchesMarkers|TestBatchPush_DuplicateTitlesFallbackToSingleCreate'
• go test -tags gms_pure_go ./internal/linear
• make test (fails in unrelated existing tests: cmd/bd TestTestServerConnection/unreachable_host and internal/storage/dolt TestPullWithAutoResolve_BranchTrackingFallbackSuccess due missing dolt executable in this environment)