feat(code): enable branch linking for local tasks

[adboio]

Problem

to build a good task wrap-up flow, we need to know which branch belongs to a given task

Changes

adds automatic branch linking for local tasks. no UI/UX around this yet, nothing uses this data yet, just storing it

agent scenarios:

scenario	current linkedBranch	agent editing on...	result
agent edits on feature branch	null	feat-a	linkedBranch = feat-a
agent edits on same linked branch	feat-a	feat-a	no change
agent switches to a new feature branch	feat-a	feaet-b	linkedBranch = feat-b
agent edits on default branch	feat-a	main	no change
agent edits on default branch, never linked	null	main	no change

ui scenarios:

scenario	result
"new branch" button	linkedBranch = new branch
"create PR" flow creates a branch	linkedBranch = new branch
"create PR" flow on existing branch	linkedBranch = new branch
branch creation fails	no change
PR creation fails	no change

How did you test this?

manually

[adboio]

• feat(code): warn on local task branch mismatch #1596
• feat(code): enable branch linking for local tasks #1595 👈 (View in Graphite)
• feat(code): add linked branch db model #1594
• main

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[jonathanlab]

I'm not sure if that's the right choice here. Maybe we can fall back to a more sensible catch all here (e.g. main/master etc) :thinking_face: