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/schema.go

@@ -17,6 +17,7 @@ CREATE TABLE IF NOT EXISTS issues (
     estimated_minutes INTEGER,
     created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     created_by TEXT DEFAULT '',
+    owner TEXT DEFAULT '',
     updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     closed_at DATETIME,
     closed_by_session TEXT DEFAULT '',