* fix(maintenance): keep titled and history-referenced prompts when pruning
prune_orphaned_prompts now also keeps prompts with a prompt_library_state
row (titled or explicit Library parents, whose state would cascade away)
and prompts that are the currently valid navigation hint of a recorded
run (execution_run_contexts positive/negative_prompt_id in the current
storage epoch whose exact raw snapshot text still matches). Stale hints
that history reads already ignore do not block pruning.
Fixes#237
* fix(maintenance): mirror unique-text hint rule and compute live hints once
The prune exempted a hinted prompt even when another prompt shared its
exact raw text, although history reads treat such a hint as dead. The
live-hint set now applies the reader's rule (current storage epoch, exact
raw text matching exactly one prompt, which is the hinted one) and lives
beside _prompt_hint in execution_contexts as LIVE_HINT_CTES.
The set is materialized once per statement and excluded with NOT IN, so
the prune no longer scans execution_run_contexts once per prompt. No
schema or index change.
Refs #237
Vito