Add branch, worktree, and remote operations #58

Problem

Concurrent session work requires explicit branch and worktree handling, and operators still need basic fetch, pull, and push flows in-app.

Scope

• Track branch and worktree creation, selection, and visibility
• Track fetch, pull, push, and remote-target visibility
• Keep cross-session repo safety explicit

Out of scope

• Full hosted Git platform management outside local repo operations
• Commit-history rendering beyond how this issue depends on it

Implementation notes

• Make worktree usage an intentional concurrency feature for the control plane
• Keep remote targets and risky operations explicit to the operator
• Align the issue with session branching and repo timeline needs

Definition of Done

• The issue defines the minimum branch, worktree, and remote operations required by the product
• Later implementation can proceed without re-deciding the operator safety model

Verification

• Review the issue against the feature spec and session branching issue

Dependencies

• Parent epic: [Epic] Add the built-in Git workspace experience #18
• Related epic: [Epic] Integrate external providers and let sessions choose one or many agents #15