Remove unused branch metadata and split_diff_view from sessions

Remove BranchParentSessionID, BranchParentPosition, and BranchCreatedAt
fields from the Session struct and Summary struct. These were stored and
persisted but never read for any logic or UI display.

The branching mechanism itself (BranchSession) remains functional - only
the provenance metadata that nothing consumed is removed.

Also drops the split_diff_view column which was already ignored in code.

Adds migration 019 to drop the columns and index from SQLite.

Author: rumpl

pkg/session/branch.go

@@ -6,7 +6,6 @@ import (
 	"maps"
 	"slices"
 	"strings"
-	"time"
 
 	"github.com/docker/docker-agent/pkg/chat"
 )
@@ -24,11 +23,6 @@ func BranchSession(parent *Session, branchAtPosition int) (*Session, error) {
 	branched := New()
 	copySessionMetadata(branched, parent, generateBranchTitle(parent.Title))
 
-	now := time.Now()
-	branched.BranchParentSessionID = parent.ID
-	branched.BranchParentPosition = &branchAtPosition
-	branched.BranchCreatedAt = &now
-
 	branched.Messages = make([]Item, 0, branchAtPosition)
 	for i := range branchAtPosition {
 		cloned, err := cloneSessionItem(parent.Messages[i])

pkg/session/branch_test.go

@@ -135,11 +135,6 @@ func TestBranchSession(t *testing.T) {
 
 		assert.NotEqual(t, parent.ID, branched.ID)
 		assert.Equal(t, "Parent Title (branched)", branched.Title)
-		assert.Equal(t, parent.ID, branched.BranchParentSessionID)
-		assert.NotNil(t, branched.BranchParentPosition)
-		assert.Equal(t, 2, *branched.BranchParentPosition)
-		assert.NotNil(t, branched.BranchCreatedAt)
-
 		assert.Len(t, branched.Messages, 2)
 		assert.Equal(t, "msg1", branched.Messages[0].Message.Message.Content)
 		assert.Equal(t, "msg2", branched.Messages[1].Message.Message.Content)

pkg/session/migrations.go

@@ -332,6 +332,18 @@ func getAllMigrations() []Migration {
 			Description: "Add index on session_items(session_id, item_type) to speed up session summary message counts",
 			UpSQL:       `CREATE INDEX IF NOT EXISTS idx_session_items_session_type ON session_items(session_id, item_type)`,
 		},
+		{
+			ID:          19,
+			Name:        "019_drop_branch_and_split_diff_columns",
+			Description: "Drop unused branch metadata columns and split_diff_view column",
+			UpSQL: `
+				DROP INDEX IF EXISTS idx_sessions_branch_parent;
+				ALTER TABLE sessions DROP COLUMN branch_parent_session_id;
+				ALTER TABLE sessions DROP COLUMN branch_parent_position;
+				ALTER TABLE sessions DROP COLUMN branch_created_at;
+				ALTER TABLE sessions DROP COLUMN split_diff_view;
+			`,
+		},
 	}
 }
 

pkg/session/session.go

@@ -113,16 +113,6 @@ type Session struct {
 	// These are shown in the model picker for easy re-selection.
 	CustomModelsUsed []string `json:"custom_models_used,omitempty"`
 
-	// BranchParentSessionID indicates this session was branched from another session.
-	BranchParentSessionID string `json:"branch_parent_session_id,omitempty"`
-
-	// BranchParentPosition is the parent session item position where this branch occurred.
-	// Only set when BranchParentSessionID is non-empty.
-	BranchParentPosition *int `json:"branch_parent_position,omitempty"`
-
-	// BranchCreatedAt is the time when this branch session was created.
-	BranchCreatedAt *time.Time `json:"branch_created_at,omitempty"`
-
 	// AgentName, when set, tells RunStream which agent to use for this session
 	// instead of reading from the shared runtime currentAgent field. This is
 	// required for background agent tasks where multiple sessions may run