feat: Add configurable polecat branch naming (#825)

feat: Add configurable polecat branch naming

Adds polecat_branch_template configuration for custom branch naming patterns.

Template variables supported:
- {user}: git config user.name  
- {year}/{month}: date (YY/MM format)
- {name}: polecat name
- {issue}: issue ID without prefix
- {description}: sanitized issue title
- {timestamp}: unique timestamp

Maintains backward compatibility - empty template uses existing format.

internal/git/git.go

@@ -558,6 +558,17 @@ func (g *Git) Remotes() ([]string, error) {
 	return strings.Split(out, "\n"), nil
 }
 
+// ConfigGet returns the value of a git config key.
+// Returns empty string if the key is not set.
+func (g *Git) ConfigGet(key string) (string, error) {
+	out, err := g.run("config", "--get", key)
+	if err != nil {
+		// git config --get returns exit code 1 if key not found
+		return "", nil
+	}
+	return out, nil
+}
+
 // Merge merges the given branch into the current branch.
 func (g *Git) Merge(branch string) error {
 	_, err := g.run("merge", branch)