backport: upstream #2392 — terminated session with open PR reads as merged

[polymath-orchestrator]

Problem

Upstream PR AgentWrapper/agent-orchestrator#2392 (fixes upstream AgentWrapper#2390), not yet merged upstream: deriveStatus's terminated branch returns merged whenever any owned PR was merged, even when the session still owns an open PR — contradicting the documented invariant ("merged/closed PRs only matter once no open PR remains"). Regression from the single-PR → multi-PR change.

This breaks our supervision sweep: a dead worker holding a stranded open PR reads as merged, so the orchestrator never respawns with --claim-pr and the PR rots.

How we got here

Our fork stays vanilla; useful not-yet-merged upstream PRs are backported as-is. Fork main is currently synced with upstream main.

Fix

Backport upstream PR AgentWrapper#2392 (head branch fix/terminated-status-open-pr):

git fetch upstream pull/2392/head:backport/2392

Rebase onto our main in a worktree, run backend gates, open a PR in our fork referencing the upstream PR. Tiny diff: backend/internal/service/session/status.go + status_test.go. Not on the sensitive-path list — normal autonomous merge gate applies.

Acceptance criteria

• Upstream PR fix(session): keep terminated status when an open PR remains AgentWrapper/agent-orchestrator#2392 applied onto our main
• Backend gates green: go build ./... && go vet ./... && go test ./...
• PR body references upstream fix(session): keep terminated status when an open PR remains AgentWrapper/agent-orchestrator#2392 as the canonical change

Context / pointers

• Upstream PR: fix(session): keep terminated status when an open PR remains AgentWrapper/agent-orchestrator#2392
• Upstream issue: bug(session): terminated session reports merged while it still owns an open PR AgentWrapper/agent-orchestrator#2390
• Priority: P2