all: s/title/slug/, adjust branch handling

There are two intertwined changes here.

First, replace title with slug, and precommit with commit-message-style.

The slug makes enough of a title, and it provides a single human-readable
identifier we can use everywhere.

Second, construct the branch name on the fly instead of storing it,
out of slug, branch prefix, and retryNumber.
This removes some duplicated data, and makes the retry loop
easier to follow and reason about.
diff --git a/loop/server/loophttp.go b/loop/server/loophttp.go
index 94d1037..5a657f8 100644
--- a/loop/server/loophttp.go
+++ b/loop/server/loophttp.go
@@ -74,7 +74,7 @@
 	MessageCount         int                           `json:"message_count"`
 	TotalUsage           *conversation.CumulativeUsage `json:"total_usage,omitempty"`
 	InitialCommit        string                        `json:"initial_commit"`
-	Title                string                        `json:"title"`
+	Slug                 string                        `json:"slug,omitempty"`
 	BranchName           string                        `json:"branch_name,omitempty"`
 	BranchPrefix         string                        `json:"branch_prefix,omitempty"`
 	Hostname             string                        `json:"hostname"`    // deprecated
@@ -1266,7 +1266,7 @@
 		WorkingDir:   getWorkingDir(),
 		// TODO: Rename this field to sketch-base?
 		InitialCommit:        s.agent.SketchGitBase(),
-		Title:                s.agent.Title(),
+		Slug:                 s.agent.Slug(),
 		BranchName:           s.agent.BranchName(),
 		BranchPrefix:         s.agent.BranchPrefix(),
 		OS:                   s.agent.OS(),