feat: Add created_by field to issues (GH#748)

Add a created_by field to track who created each issue, similar to how
comments have an author field.

- Add CreatedBy string field to Issue struct
- Add migration 029 to add created_by column to issues table
- Update all SELECT/INSERT/Scan statements across storage layer
- Populate created_by in bd create from actor chain
- Display created_by in bd show output

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/schema.go

@@ -16,6 +16,7 @@ CREATE TABLE IF NOT EXISTS issues (
     assignee TEXT,
     estimated_minutes INTEGER,
     created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    created_by TEXT DEFAULT '',
     updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     closed_at DATETIME,
     external_ref TEXT,