Salvages #23302 by @Bartok9. Four independent one-area fixes:
1. kanban boards delete alias now hard-deletes (not archives) — the
alias didn't carry --delete, so getattr(args, 'delete', False)
returned False. Detect boards_action=='delete' explicitly.
2. Gateway auto-title failures no longer leak as user-visible
warnings — debug-log only since they're not actionable.
3. Background process completion notification snaps truncation to
the next newline boundary, prepends a marker when content is
dropped.
4. _cprint() schedules the run_in_terminal coroutine via
asyncio.ensure_future so output isn't silently dropped from
background threads (fixes#23185 Bug A). Skips the
double-print fallback that would fire for mock paths.
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