Continuation of an existing Issue branch #6

Parent

#2

What to build

Continuation (ADR-0002). Issue branch selection looks at the remote Issue branches for the issue and all their PRs, whatever their state, using one git ls-remote and one gh pr list --state all. If the highest-numbered Issue branch exists on origin with no PR or an open PR, the run continues it:

• The worktree checks out the existing Issue branch from origin, not a new one from the Base branch.
• The session gets the continuation prompt from the spec: the "update PR " form if an open PR exists, otherwise the create-PR form.
• With an open PR, that PR's base becomes the Base branch (the review fixed point and the target), and the Base branch pre-flight checks apply to it. If it differs from the branch checked out in the launch directory, stderr says so.
• A local Issue branch in the launch repo that points somewhere other than its origin copy → error before anything is created.

Acceptance criteria

• Existing issue-<n> on origin with no PR → continued; the agent's new commits sit on top of the existing ones; the continuation prompt uses the create-PR form
• Existing issue-<n> with an open PR → continued; the continuation prompt names the PR URL and uses the update form; the same PR is the result
• Open PR base main while develop is checked out → the Base branch is main in the prompt and checks, with a note on stderr
• A local issue-<n> that differs from origin/issue-<n> → error, and nothing created
• A Continuation of a Failed run's branch (ending in a failure commit) works
• Selection uses the remote branches plus PR history in at most two network calls

Blocked by

• Walking skeleton: a fresh run on the happy path #3