CI caught tests/cli/test_cli_new_session.py asserting that /new keeps
the old session row when conversation history exists in memory. The
live transcript is authoritative: a session whose messages haven't
flushed to the DB yet (or whose flush failed) must not be pruned.
Guard _discard_session_if_empty on self.conversation_history and pin
the behavior with a test.
Port from google-gemini/gemini-cli#27770: starting the CLI and
immediately quitting (or rotating with /new, /clear) left an empty
untitled session row behind. These ghost rows pile up in /resume,
`hermes sessions list`, and the in-chat recent-sessions browser.
- SessionDB.delete_session_if_empty(): transactional check-and-delete
that only removes rows with no messages, no title, and no child
sessions (delegate subagent parents are preserved). Also removes
on-disk transcript files via the existing _remove_session_files.
- HermesCLI._discard_session_if_empty(): thin wrapper, wired into the
cli_close shutdown path and the new_session() rotation path.
Skipped when /exit --delete already handles removal.
Unlike the one-shot prune_empty_ghost_sessions migration (TUI-only,
24h-old rows), this prevents new ghost rows from accumulating at the
moment they would be created.
