Fix template rendering: always render directives and cache git diff

[Copilot]

Template directives ({{.File}}, {{.GitDiff}}) were only rendered when --var was passed, because rendering was gated on vars being non-nil. Additionally, multiple {{.GitDiff}} occurrences triggered redundant git diff subprocess calls.

• util.cr: PromptInput.resolve now always calls TemplateRenderer.render, passing an empty hash when no vars are provided
• retry.cr: Same fix — rendering no longer conditional on vars presence
• template_renderer.cr: Compute git diff once per render call and reuse via String#gsub instead of invoking get_git_diff per regex match

# Before: directives silently ignored without --var
cjules new --template my-template "do something"

# After: {{.File "path"}} and {{.GitDiff}} resolve correctly regardless of --var