Function formatBundleForPrompt

Produce a chat-prompt fenced representation of the bundle.

• The manifest always lands first, fully serialised.
• At most maxFiles code/widget files follow.
• Each file is capped at maxFileChars with a [truncated] marker.
• Widget files (path starts with widgetPathPrefix) get the ifc-extension-widget fence; everything else gets ifc-extension-code.

Pure: no side effects, deterministic for a given input.