Partial salvage of #100887 by @jwilson411. The restore-path half (raise
TranscriptReadError from load_transcript, fail the turn closed) already landed
via #100910; this keeps the complementary half: every slash-command handler and
platform helper that reads the transcript now catches TranscriptReadError and
tells the user the history exists but is unreadable, instead of letting the
exception reach the dispatch wrapper, which logs it and sends no reply.
(cherry picked from commit 2a132c903f51c8d21f6eea64ddeef688c9619a18, run.py/session.py
hunks dropped as already on main; notice-path tests replaced accordingly)
dd3e6ed932