feat: add type: event for operational state changes (bd-ecmd)

Adds support for event beads that capture operational state transitions
as immutable records. Events are a new issue type with fields:
- event_kind: namespaced category (patrol.muted, agent.started)
- actor: entity URI who caused the event
- target: entity URI or bead ID affected
- payload: event-specific JSON data

This enables:
- bd activity --follow showing events
- bd list --type=event --target=agent:deacon
- Full audit trail for operational state
- HOP-compatible transaction records

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

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

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

internal/storage/sqlite/migrations_test.go

@@ -503,9 +503,13 @@ func TestMigrateContentHashColumn(t *testing.T) {
 				role_type TEXT DEFAULT '',
 				rig TEXT DEFAULT '',
 				mol_type TEXT DEFAULT '',
+				event_kind TEXT DEFAULT '',
+				actor TEXT DEFAULT '',
+				target TEXT DEFAULT '',
+				payload TEXT DEFAULT '',
 				CHECK ((status = 'closed') = (closed_at IS NOT NULL))
 			);
-			INSERT INTO issues SELECT id, title, description, design, acceptance_criteria, notes, status, priority, issue_type, assignee, estimated_minutes, created_at, '', updated_at, closed_at, external_ref, compaction_level, compacted_at, original_size, compacted_at_commit, source_repo, '', NULL, '', '', '', '', 0, 0, 0, '', '', '', '', '', '', 0, '', '', '', '', NULL, '', '', '' FROM issues_backup;
+			INSERT INTO issues SELECT id, title, description, design, acceptance_criteria, notes, status, priority, issue_type, assignee, estimated_minutes, created_at, '', updated_at, closed_at, external_ref, compaction_level, compacted_at, original_size, compacted_at_commit, source_repo, '', NULL, '', '', '', '', 0, 0, 0, '', '', '', '', '', '', 0, '', '', '', '', NULL, '', '', '', '', '', '', '' FROM issues_backup;
 			DROP TABLE issues_backup;
 		`)
 		if err != nil {