ConversationSettings clamped prune_excess to minimum=100, so the
helper's prune_excess=50 was silently raised to 100, making
trigger_line_count=200 exactly equal to the 200 lines added in the
robot test. The > check never fired, pruning never ran, and both
prune-trigger and prune-note-inserted failed.
Fixes:
- Lower prune_excess minimum from 100 to 10 in _normalise_positive so
values like 50 are accepted without clamping.
- Change range(20) to range(21) in cmd_prune_trigger and
cmd_prune_note_inserted: 21 blocks x 10 lines = 210 lines triggers
pruning twice (at block 15 and block 20), leaving total_lines=91
after the final prune event which satisfies the <= 100 assertion.
ISSUES CLOSED: #6350