Make feature branch prefix configurable

[eslerm]

• Replace hardcoded branch prefix with branch_prefix config field
• Falls back to git config user.name, then no prefix

[mgreau]

Two related issues on the git config user.name fallback in GetBranchPrefix.

[mgreau]

Two issues here:

1. exec.Command has no timeout. Every other subprocess in this codebase now uses withTimeout / exec.CommandContext. This call can hang indefinitely if git is slow or the working directory is unusual.

2. user.name can contain spaces (e.g. "John Doe"), producing branch names like John Doe/feature that break many tools and CLIs.

Suggestion fixes both — wraps the call in a 5s context and replaces spaces with hyphens. Also needs "context" added to the import block.

Suggested change

	// Try git config
	out, err := exec.Command("git", "config", "user.name").Output()
	if err == nil {
	name := strings.TrimSpace(string(out))
	// Try git config user.name; replace spaces so the prefix is branch-safe.
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	out, err := exec.CommandContext(ctx, "git", "config", "user.name").Output()
	if err == nil {
	name := strings.ReplaceAll(strings.TrimSpace(string(out)), " ", "-")

[mgreau]

Superseded by #7, which incorporates the review feedback (5s timeout on git config call, spaces→hyphens in user.name).