refactor: Organize Issue struct with section comments (bd-5l59)
Group 50+ fields into 15 logical sections with clear separators:
- Core Identification (ID, ContentHash)
- Issue Content (Title, Description, Design, AcceptanceCriteria, Notes)
- Status & Workflow (Status, Priority, IssueType)
- Assignment (Assignee, EstimatedMinutes)
- Timestamps (CreatedAt, CreatedBy, UpdatedAt, ClosedAt, CloseReason)
- External Integration (ExternalRef)
- Compaction Metadata
- Internal Routing (not exported)
- Relational Data (Labels, Dependencies, Comments)
- Tombstone Fields (soft-delete)
- Messaging Fields (inter-agent communication)
- Context Markers (Pinned, IsTemplate)
- Bonding Fields (compound molecule lineage)
- HOP Fields (entity tracking)
- Gate Fields (async coordination)
- Source Tracing Fields
- Agent Identity Fields
All tests pass, JSON serialization unchanged.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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