GitHub App comment author is derived from App name instead of slug

[vrivellino]

Summary

When argo-diff is configured as a GitHub App, getCommentUser() derives the bot login from the
App's display name instead of its slug. If the App name is not already slug-shaped, the
computed login never matches the real comment author, and argo-diff stops recognizing its own
previous comments.

Where

internal/github/comment.go:150

commentLogin = *app.Name + "[bot]"

Why it's wrong

GitHub builds a bot's login from the App slug — the URL-friendly form of the name (lowercased,
spaces replaced with hyphens) — not from the display name.

App display name	Real comment author	Value computed today
argo-diff	argo-diff[bot]	argo-diff[bot] ✅
Argo Diff	argo-diff[bot]	Argo Diff[bot] ❌
ArgoDiff Prod	argodiff-prod[bot]	ArgoDiff Prod[bot] ❌

go-github already exposes the correct field — App.Slug alongside App.Name
(github/apps.go:22).

Impact

getExistingComments() filters candidate comments with:

if isGithubAction || *c.User.Login == commentLogin {

(internal/github/comment.go:279)

When commentLogin is wrong, that comparison never matches. The consequences:

• argo-diff never finds its previous comment on a PR.
• Every event creates a new full-size diff comment instead of editing the existing one.
• The [Outdated argo-diff content] cleanup path never runs, so stale comments accumulate.

The symptom is duplicate comments on every push to a PR.

This is latent rather than universal because README.md recommends naming the App argo-diff,
which is already slug-shaped. Any other name silently breaks comment reuse. Nothing in the setup
docs warns that the name matters.

Suggested fix

Use the slug, and keep the name only as a fallback:

if app == nil || (app.Slug == nil && app.Name == nil) {
	log.Error().Msg("Empty app returned - not sure how I got here")
	return fmt.Errorf("empty app info")
}
appLogin := app.GetSlug()
if appLogin == "" {
	log.Warn().Msg("Github App slug is empty - falling back to app name")
	appLogin = app.GetName()
}
mux.Lock()
commentLogin = appLogin + "[bot]"
mux.Unlock()

Note the nil check at internal/github/comment.go:145 also needs updating, since it currently
guards only app.Name.

Test coverage gap

There is no test for the GitHub App identity path. internal/github/github_testdata/ has
payload-user.json for the PAT path but no /app fixture, and no test exercises
commentClientIsApp = true. A fixture where slug and name differ would pin this behavior down.