generate-help with a nonexistent subcommand fabricates a plausible help page

[baijum]

Summary

When find-command returns #f, generate-help app "name" silently falls back to the app's description and specs (lib/kaappi/cli.sld:233-246) while still printing the nonexistent sub-name in the header — producing a confident-looking page for a command that does not exist.

Repro

App with only the build command:

$ (generate-help app "nope")
t nope — T

Usage: t nope <command>

Commands:
  build                     B

Expected

An error (or at least the app-level help), not a fabricated subcommand page. Not reachable via run-cli (it only passes real command names), but generate-help is exported API.

Context

Found during a multi-agent code review (2026-09-06); confirmed empirically on Kaappi v0.26.2.