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/loop/server/loophttp.go b/loop/server/loophttp.go
index b186760..a89557d 100644
--- a/loop/server/loophttp.go
+++ b/loop/server/loophttp.go
@@ -1112,12 +1112,13 @@
 	totalUsage := s.agent.TotalUsage()
 
 	return State{
-		StateVersion:         2,
-		MessageCount:         serverMessageCount,
-		TotalUsage:           &totalUsage,
-		Hostname:             s.hostname,
-		WorkingDir:           getWorkingDir(),
-		InitialCommit:        s.agent.InitialCommit(),
+		StateVersion: 2,
+		MessageCount: serverMessageCount,
+		TotalUsage:   &totalUsage,
+		Hostname:     s.hostname,
+		WorkingDir:   getWorkingDir(),
+		// TODO: Rename this field to sketch-base?
+		InitialCommit:        s.agent.SketchGitBase(),
 		Title:                s.agent.Title(),
 		BranchName:           s.agent.BranchName(),
 		OS:                   s.agent.OS(),