run fetch before push operations

[skarim]

Adds a targeted git fetch of active stack branches before every git push call. This fixes --force-with-lease rejections for users with shallow clones, where remote tracking refs may be stale or missing.

Changes

• New FetchBranches(remote, branches) in the git layer — fetches only the specific branches being pushed
• push — fetch active branches before push (also switches to non-atomic push)
• submit — fetch active branches before the per-branch push loop
• link — fetch specified branches before push

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[Copilot]

Pull request overview

This PR adds a branch-scoped fetch helper to the git layer and wires it into stack push flows so branch refs are refreshed immediately before pushing. It touches the CLI commands that publish stack branches (push, submit, and link) plus their test coverage.

Changes:

• Add FetchBranches(remote, branches) to the git ops interface, default implementation, wrapper, and test mock.
• Run the new fetch step before push operations in submit, push, and link.
• Add tests for fetch-before-push ordering, and update push expectations around atomic push behavior.

Show a summary per file

File	Description
internal/git/mock_ops.go	Extends the git mock with a branch-scoped fetch hook for tests.
internal/git/gitops.go	Adds the concrete FetchBranches implementation to the git ops layer.
internal/git/git.go	Exposes the new fetch helper through the package-level API.
cmd/submit.go	Fetches active remote branches before sequential submit pushes.
cmd/submit_test.go	Verifies submit performs fetch before pushing stack branches.
cmd/push.go	Fetches active branches before push and changes push atomicity.
cmd/push_test.go	Adds fetch-before-push coverage and updates push behavior assertions.
cmd/link.go	Fetches candidate branches before pushing them during link.
cmd/link_test.go	Verifies link performs fetch before push.

Copilot's findings

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• Files reviewed: 9/9 changed files
• Comments generated: 3

[skarim]

Addressed review feedback.