feat(code): add linked branch db model (#1594)

## Problem

there is no persistent store that says "this task belongs to this branch"

this makes it very difficult to build a good UX around the end-of-task workflow

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## Changes

adds infra for a new `linkedBranch `field on the workspace model

no behavior changes in this PR, just setting things up

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## How did you test this?

manually (upstack)

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Author: adboio

apps/code/src/main/db/migrations/0005_youthful_scarlet_spider.sql

@@ -0,0 +1 @@
+ALTER TABLE `workspaces` ADD `linked_branch` text;