fix(code): dedupe suggested branch names

[adboio]

Problem

1. the suggest branch names are not deduped
2. "get all branches" filters out code-managed branches (by prefix) which makes deduping harder and i think is generally confusing

Changes

1. adds deduping to suggested branch names
2. removes branch filtering
3. prefetches all branches on task focus, instead of waiting for the branch selector to be opened

How did you test this?

manually

[adboio]

• feat(code): disable editing in diff viewer #1395 : 2 dependent PRs (#1397 , #1409 )
• feat(code): unified PR creation workflow #1377
• send full diff to PR title/body generator #1394
• fix(code): dedupe suggested branch names #1393 👈 (View in Graphite)
• fix(code): increase diff scan limit #1392
• main

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[adboio]

Merge activity

• Mar 31, 8:47 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Mar 31, 8:49 PM UTC: Graphite rebased this pull request as part of a merge.
• Mar 31, 8:56 PM UTC: Graphite couldn't merge this PR because it was not satisfying all requirements (PR does not have required approvals).

The base branch was changed.