feat: add owner field for human attribution in HOP CV chains

Add 'owner' field to Issue struct for tracking the human responsible
for the issue, distinct from 'created_by' which tracks the executor.
Owner is populated from git author email (GIT_AUTHOR_EMAIL or git
config user.email), per Decision 008 for CV accumulation.

Changes:
- Add Owner field to types.Issue with omitempty JSON tag
- Include Owner in content hash computation
- Add owner column migration (036_owner_column.go)
- Update all SQL queries to include owner field
- Add getOwner() helper using git author email fallback chain
- Populate owner in bd create command
- Add owner to RPC CreateArgs protocol

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

Executed-By: beads/crew/dave
Rig: beads
Role: crew

internal/storage/sqlite/migrations.go

@@ -52,6 +52,7 @@ var migrationsList = []Migration{
 	{"event_fields", migrations.MigrateEventFields},
 	{"closed_by_session_column", migrations.MigrateClosedBySessionColumn},
 	{"due_defer_columns", migrations.MigrateDueDeferColumns},
+	{"owner_column", migrations.MigrateOwnerColumn},
 }
 
 // MigrationInfo contains metadata about a migration for inspection
@@ -111,6 +112,7 @@ func getMigrationDescription(name string) string {
 		"event_fields":                 "Adds event fields (event_kind, actor, target, payload) for operational state change beads",
 		"closed_by_session_column":     "Adds closed_by_session column for tracking which Claude Code session closed an issue",
 		"due_defer_columns":            "Adds due_at and defer_until columns for time-based task scheduling (GH#820)",
+		"owner_column":                 "Adds owner column for human attribution in HOP CV chains (Decision 008)",
 	}
 
 	if desc, ok := descriptions[name]; ok {