_prune_filter_where documents title_like / model_like / branch_like as
"case-insensitive substring matches", and the CLI confirmation renders them
as "title contains 'X'". They were bound straight into a bare LIKE, so `_`
matched any single character and `%` any run.
The builder backs prune_sessions(), which deletes session rows and their
on-disk transcripts, so the over-match is unrecoverable: pruning
title_like="user_auth" also destroys "user-auth", "userXauth" and
"user auth". `_` is not exotic here -- git branch names and session titles
carry it routinely.
Escape the operator's needle and add ESCAPE '\' to the three clauses, the
same convention the rest of this file already uses for LIKE queries. Match
direction is unchanged for needles without wildcards.
Left alone: _cwd_prefix_clause has the same unescaped shape but is shared
by four call sites beyond prune, so it is a separate change.
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