agent: move "sketch-base" into git

The agent's notion of "initial commit" is kind of special, in that it
is used as the "base" for a bunch of git operations. It's hard for
the user to change (we only provide a workflow via restart), yet
sometimes you want to do just that.

So, instead we put it as data inside of it, named as a tag sketch-base.
It's abusing tags, but branches are no better.
diff --git a/termui/termui.go b/termui/termui.go
index 1575473..6dc5f87 100644
--- a/termui/termui.go
+++ b/termui/termui.go
@@ -262,10 +262,11 @@
 					branches = append(branches, branch)
 				}
 
-				initialCommitRef := getShortSHA(ui.agent.InitialCommit())
+				initialCommitRef := getShortSHA(ui.agent.SketchGitBase())
 				if len(branches) == 1 {
 					ui.AppendSystemMessage("\nšŸ”„ Branch pushed during session: %s", branches[0])
 					ui.AppendSystemMessage("šŸ’ To add those changes to your branch: git cherry-pick %s..%s", initialCommitRef, branches[0])
+					ui.AppendSystemMessage("šŸ”€                                   or git merge %s", branches[0])
 				} else {
 					ui.AppendSystemMessage("\nšŸ”„ Branches pushed during session:")
 					for _, branch := range branches {
@@ -275,6 +276,14 @@
 					for _, branch := range branches {
 						ui.AppendSystemMessage("git cherry-pick %s..%s", initialCommitRef, branch)
 					}
+					ui.AppendSystemMessage("\nšŸ’ To add all those changes to your branch:")
+					for _, branch := range branches {
+						ui.AppendSystemMessage("git cherry-pick %s..%s", initialCommitRef, branch)
+					}
+					ui.AppendSystemMessage("\nšŸ”€                              or:")
+					for _, branch := range branches {
+						ui.AppendSystemMessage("git merge %s", branch)
+					}
 				}
 			}
 			ui.mu.Unlock()