Normalize GitHub issue traceability identifiers so created links sync correctly

[Brad-Edwards]

Raised from the 2026-03-28 full-codebase architecture and code-quality review.

Problem

createIssueFromRequirement() stores GitHub issue traceability identifiers as #<number>, while the sync path later assumes the identifier is a raw integer string and parses it with Integer.parseInt(...).

Evidence

• backend/src/main/java/com/keplerops/groundcontrol/domain/requirements/service/GitHubIssueSyncService.java
• backend/src/test/java/com/keplerops/groundcontrol/integration/SyncIntegrationTest.java
• backend/src/test/java/com/keplerops/groundcontrol/unit/domain/GitHubIssueSyncServiceTest.java

Why this matters

The product path that creates issues from requirements cannot reliably round-trip through the sync/enrichment path. The existing integration coverage only validates manually-stored raw numeric identifiers, not the identifier shape the app itself writes.

Definition of done

• Choose one canonical format for GitHub issue identifiers.
• Make issue creation and sync/update logic use that same format.
• Add integration coverage for the exact create-from-requirement path.
• Ensure previously-created links either remain supported or are migrated safely.