Migrate GitOps PR + chore-issue flows from `gh` CLI to go-github API

[antoinetoussaint-byte]

Summary

The GitOps promotion flow and the "core is behind" chore-issue flow shell out to the gh CLI for pull-request and issue operations. These are pure GitHub REST API calls; move them to github.com/google/go-github/v89 (already a direct dependency, go.mod:22).

Lower priority than the agent-release migration (#457) because these paths run in developer/CI contexts where gh is more reliably present — but consolidating onto one client removes the last hard gh dependencies and makes error handling/rate-limit behavior consistent.

What uses gh today

Pull requests — pkg/gitops/publish.go:

• gh pr list (:1208)
• gh pr edit (:1230)
• gh pr create (:1236)
• gh pr view — verifyPullRequest (:1250)

PR reads — pkg/gitops/observe.go:

• gh pr view (:726)

Chore issue — cmd/status/release.go:

• gh issue create (:266) — opens the "core is behind" issue.

Proposed change

Replace with go-github calls, reusing the shared client/token helper (see #457 — ideally a pkg/github with newGitHubClient + githubToken):

• gh pr list → client.PullRequests.List (filter by head/base/state)
• gh pr create → client.PullRequests.Create
• gh pr edit → client.PullRequests.Edit
• gh pr view → client.PullRequests.Get
• gh issue create → client.Issues.Create

Note — keep git for git

pkg/gitops/publish.go / observe.go / remote.go also push branches via the gitCommand git shell-out (clone, clone --mirror, push --porcelain). Those are genuine git operations and should stay on the git binary — this issue only covers the PR/issue API calls.

Acceptance

• PR list/create/edit/view and chore-issue creation go through go-github, no gh pr * / gh issue * shell-outs.
• git branch pushes untouched (still gitCommand).
• Shared client/token helper reused (not a third copy of the plumbing).

Part of #460.