feat: complete bd-kwro messaging & knowledge graph epic

- Add bd cleanup --ephemeral flag for transient message cleanup (bd-kwro.9)
- Add Ephemeral filter to IssueFilter type
- Add ephemeral filtering to SQLite storage queries

Tests (bd-kwro.10):
- Add internal/hooks/hooks_test.go for hook system
- Add cmd/bd/mail_test.go for mail commands
- Add internal/storage/sqlite/graph_links_test.go for graph links

Documentation (bd-kwro.11):
- Add docs/messaging.md for full messaging reference
- Add docs/graph-links.md for graph link types
- Update AGENTS.md with inter-agent messaging section
- Update CHANGELOG.md with all bd-kwro features

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

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

internal/storage/sqlite/queries.go

@@ -1552,6 +1552,15 @@ func (s *SQLiteStorage) SearchIssues(ctx context.Context, query string, filter t
 		whereClauses = append(whereClauses, fmt.Sprintf("id IN (%s)", strings.Join(placeholders, ", ")))
 	}
 
+	// Ephemeral filtering (bd-kwro.9)
+	if filter.Ephemeral != nil {
+		if *filter.Ephemeral {
+			whereClauses = append(whereClauses, "ephemeral = 1")
+		} else {
+			whereClauses = append(whereClauses, "(ephemeral = 0 OR ephemeral IS NULL)")
+		}
+	}
+
 	whereSQL := ""
 	if len(whereClauses) > 0 {
 		whereSQL = "WHERE " + strings.Join(whereClauses, " AND ")