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/types/types.go

@@ -30,6 +30,7 @@ type Issue struct {
 
 	// ===== Assignment =====
 	Assignee         string `json:"assignee,omitempty"`
+	Owner            string `json:"owner,omitempty"` // Human owner for CV attribution (git author email)
 	EstimatedMinutes *int   `json:"estimated_minutes,omitempty"`
 
 	// ===== Timestamps =====
@@ -133,6 +134,7 @@ func (i *Issue) ComputeContentHash() string {
 	w.int(i.Priority)
 	w.str(string(i.IssueType))
 	w.str(i.Assignee)
+	w.str(i.Owner)
 	w.str(i.CreatedBy)
 
 	// Optional fields