feat(code): replace in-chat diffs with pierre/diffs

[adboio]

Problem

as part of #1410, i replaced codemirror with @pierre/diffs in the changed file diff views

but the in-chat diffs still use codemirror

Changes

for consistency mostly, replacing the in-chat diff view with the same thing, and some related cleanup

How did you test this?

manually

[adboio]

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• feat(code): add comment-to-agent in diff viewer #1416
• fix(code): fix timestamp hover #1415
• feat(code): replace in-chat diffs with pierre/diffs #1414 👈 (View in Graphite)
• feat(code): replace diff editor with unified review panel #1410
• feat(code): add git diff head query #1412
• get full list of changed files #1411
• chore: extract some utils #1409
• feat(code): disable editing in diff viewer #1395 : 1 other dependent PR (#1397 )
• feat(code): unified PR creation workflow #1377
• send full diff to PR title/body generator #1394
• fix(code): dedupe suggested branch names #1393
• fix(code): increase diff scan limit #1392
• main

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