feat(schema): add messaging fields for bd-kwro epic

- Add TypeMessage issue type for inter-agent communication
- Add 6 new Issue fields: Sender, Ephemeral, RepliesTo, RelatesTo,
  DuplicateOf, SupersededBy
- Add 4 new dependency types: replies-to, relates-to, duplicates, supersedes
- Create migration 019_messaging_fields with indexes
- Update all CRUD operations across storage layer
- Fix reset_test.go to use correct function names
- Fix redundant newline lint error in sync.go

Closes: bd-kwro.1

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

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

internal/storage/sqlite/migrations/019_messaging_fields.go

@@ -0,0 +1,69 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+// MigrateMessagingFields adds messaging and graph link support columns to the issues table.
+// These columns support inter-agent communication (bd-kwro):
+// - sender: who sent this message
+// - ephemeral: can be bulk-deleted when closed
+// - replies_to: issue ID for conversation threading
+// - relates_to: JSON array of issue IDs for knowledge graph edges
+// - duplicate_of: canonical issue ID (this is a duplicate)
+// - superseded_by: replacement issue ID (this is obsolete)
+func MigrateMessagingFields(db *sql.DB) error {
+	columns := []struct {
+		name       string
+		definition string
+	}{
+		{"sender", "TEXT DEFAULT ''"},
+		{"ephemeral", "INTEGER DEFAULT 0"},
+		{"replies_to", "TEXT DEFAULT ''"},
+		{"relates_to", "TEXT DEFAULT ''"},
+		{"duplicate_of", "TEXT DEFAULT ''"},
+		{"superseded_by", "TEXT DEFAULT ''"},
+	}
+
+	for _, col := range columns {
+		var columnExists bool
+		err := db.QueryRow(`
+			SELECT COUNT(*) > 0
+			FROM pragma_table_info('issues')
+			WHERE name = ?
+		`, col.name).Scan(&columnExists)
+		if err != nil {
+			return fmt.Errorf("failed to check %s column: %w", col.name, err)
+		}
+
+		if columnExists {
+			continue
+		}
+
+		_, err = db.Exec(fmt.Sprintf(`ALTER TABLE issues ADD COLUMN %s %s`, col.name, col.definition))
+		if err != nil {
+			return fmt.Errorf("failed to add %s column: %w", col.name, err)
+		}
+	}
+
+	// Add index for ephemeral issues (for efficient cleanup queries)
+	_, err := db.Exec(`CREATE INDEX IF NOT EXISTS idx_issues_ephemeral ON issues(ephemeral) WHERE ephemeral = 1`)
+	if err != nil {
+		return fmt.Errorf("failed to create ephemeral index: %w", err)
+	}
+
+	// Add index for sender (for efficient inbox queries)
+	_, err = db.Exec(`CREATE INDEX IF NOT EXISTS idx_issues_sender ON issues(sender) WHERE sender != ''`)
+	if err != nil {
+		return fmt.Errorf("failed to create sender index: %w", err)
+	}
+
+	// Add index for replies_to (for efficient thread queries)
+	_, err = db.Exec(`CREATE INDEX IF NOT EXISTS idx_issues_replies_to ON issues(replies_to) WHERE replies_to != ''`)
+	if err != nil {
+		return fmt.Errorf("failed to create replies_to index: %w", err)
+	}
+
+	return nil
+}