Three small fixes from Opus review of the merged stage diff:
1. Strip 9 orphan wiki_* i18n keys (72 lines) from PR #1342 — leaked
from a different branch, zero references outside i18n.js.
2. /branch endpoint: reject non-string session_id with explicit 400
(was raising TypeError → generic 500 from get_session()).
3. /branch endpoint: reject negative keep_count with explicit 400
(Python slice semantics on negative produces 'all but last N',
confusing fork behavior).
Plus tests/test_v050253_opus_followups.py — 3 regression tests pinning
all three fixes.
Verified: 3558 pytest passing.
67193faf38