Gateway /undo was wired into every platform but still ran the old
single-turn hard-truncate. Now it matches the CLI/TUI: /undo [N] backs
up N user turns (default 1, clamps to oldest), soft-deletes the
truncated rows on disk (active=0, kept for audit, hidden from re-prompts
and search) via SessionDB.rewind_to_message, evicts the cached agent so
the next turn rebuilds from the active-only transcript (the gateway's
equivalent of the CLI's in-place history surgery + memory invalidation),
and echoes the backed-up message text so the user can copy/edit and
resend — platforms have no editable composer to prefill.
- gateway/session.py: SessionStore.rewind_session(session_id, n) wraps
the soft-delete primitive; load_transcript already returns active-only
- gateway/run.py: _handle_undo_command parses [N], calls rewind_session,
evicts the agent, echoes target text; confirm-prompt detail is count-aware
- locales: undo.removed gains {turns}; new undo.invalid_count, all 16 langs
- tests: tests/gateway/test_undo_rewind_session.py (6 cases)
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