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/termui/termui.go b/termui/termui.go
index 24f663b..bbe04b4 100644
--- a/termui/termui.go
+++ b/termui/termui.go
@@ -44,10 +44,10 @@
  ⌨️  {{.input.path -}}
 {{else if eq .msg.ToolName "done" -}}
 {{/* nothing to show here, the agent will write more in its next message */}}
-{{else if eq .msg.ToolName "title" -}}
-🏷️  {{.input.title}}
-{{else if eq .msg.ToolName "precommit" -}}
-🌱 git branch: {{.branch_prefix}}{{.input.branch_name}}
+{{else if eq .msg.ToolName "set-slug" -}}
+🐌 {{.input.slug}}
+{{else if eq .msg.ToolName "commit-message-style" -}}
+🌱 learn git commit message style
 {{else if eq .msg.ToolName "about_sketch" -}}
 📚 About Sketch
 {{else if eq .msg.ToolName "codereview" -}}