fix(linear): dedupe batch creates via idempotency pre-check

[cursor]

Summary

• prevent duplicate Linear issue creation in batch sync by pre-checking each issue's idempotency marker before calling issueBatchCreate
• reuse already-created Linear issues discovered by marker search and only batch-create unresolved inputs
• keep ambiguous-failure recovery scoped to the pending subset after pre-check filtering

Bug and impact

When a prior sync created a Linear issue but failed to persist local external_ref (or returned an ambiguous batch failure), the next sync retried issueBatchCreate without checking existing markers. That could create duplicate external issues for the same bead.

Validation

• CGO_ENABLED=1 go test -tags gms_pure_go ./internal/linear
• make test (still fails on pre-existing/environmental suites unrelated to this change: cmd/bd unreachable-host assertion and internal/storage/dolt missing dolt binary in PATH)