fix(linear): avoid repeated batch updates for unchanged structured issues

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Summary

Fixes a critical batch-sync regression where unchanged Linear issues could be re-updated on every sync when AcceptanceCriteria, Design, or Notes were present.

Bug + impact

When batch push receives issues whose Description was already formatted by engine hooks, skip comparison re-ran formatting and produced a different string. This made unchanged issues look modified, causing repeated updates and avoidable rate-limit pressure that can break large sync runs.

Root cause

BatchPush used PushFieldsEqual(local, remote, ...), which always calls BuildLinearDescription(local). In batch mode, local.Description may already be pre-formatted while structured fields remain populated, so re-formatting duplicated appended sections in-memory and defeated skip detection.

Fix

• Added a batch-safe description normalizer that detects already-formatted payloads.
• Used that normalized description for:
  • skip comparisons,
  • single-create duplicate-title path,
  • batch-create payload construction,
  • update payload description field.
• Added regression test: TestBatchPush_SkipsUnchangedIssue_WithStructuredFields.

Validation

• CGO_ENABLED=1 go test -tags gms_pure_go ./internal/linear -run 'TestBatchPush_SkipsUnchangedIssue|TestBatchPush_SkipsUnchangedIssue_WithStructuredFields|TestBatchPush_ForceBypassesSkip'
• make test (fails in unrelated pre-existing environment-dependent tests in cmd/bd and internal/storage/dolt, including missing dolt binary and existing network-assumption assertions)