fix(chat): commit divider polish + persistent boundary

[huhamhire]

Two follow-up fixes to the ChatPane commit divider (from #216).

1. Chip gap / alignment + real commit message on hover (900e882)

• The chip's commit icon and short sha were flush and vertically misaligned (the chip mixin sets no gap). Add a small gap and make the svg a block so it aligns to the sha's line box.
• The hover tooltip showed a generic "code changed" string. Show the new head commit's actual message instead (resolved via diff:listCommits), falling back to the short sha. Removes the chatPane.commitDividerTitle i18n key from all four locales.

2. Keep the divider as a persistent boundary (ec2751f)

The bottom "stale head" divider vanished once a run was executed against the new commit (head then equalled the last run's headSha → nothing "stale"). Now mark every point where the reviewed commit changes:

• a divider before a run whose headSha differs from the previous run's — a durable boundary between the old-commit runs above and the new-commit runs below (no longer disappears after reviewing the new code);
• a trailing bottom divider only while the head has advanced past the last run and no run against it exists yet (new commit not reviewed, incl. while a run is in flight).

Drops the "suppress while a run is active" rule so it never flickers away. Tooltip messages come from a single sha→message map (diff:listCommits), so every divider resolves its own commit message.

Checks

lint + typecheck + build (desktop) pass locally.