Episode notes for a private feed opened by link embed the feed's secret URL #449

Problem. An episode opened through an obsidian://podnotes link for a private feed that is not subscribed keeps the feed's credential-bearing URL. Creating its episode note writes that URL into the vault twice: inside the {{episodelink}} resume link (the default note template uses it) and in the note's feedUrl frontmatter. A synced vault then carries the secret.

Scope. This predates #447. #447 only hides the episode menu's "Copy episode link" in this case, so the clipboard path is closed; the template and frontmatter paths are not, because changing buildEpisodeResumeLink also changes the public API's output.

Expected. No credential-bearing URL reaches a note. For example, buildEpisodeResumeLink returns an empty link (or a placeholder-based one) when its target carries a secret, and feedUrl frontmatter uses the private feed placeholder, with the API change recorded.

Found by UI1a's review (interrogate round 2).