Distrust origin/<branch> when prune's fetch fails

A stale remote-tracking ref survives a failed fetch and the next successful
--prune deletes it, so it cannot prove a branch's commits survive upstream.
Record whether the pruning fetch succeeded and gate the origin/<branch>
existence and containment proofs on it; a failed fetch removes the worktree and
keeps the branch. Local-object proofs and the merged head SHA are unaffected.

cmd/agentws/prune.go

@@ -40,6 +40,9 @@ type repoCtx struct {
 	defBranch string
 	prs       map[string]agent.PullRequest
 	prsKnown  bool
+	// fetched records that this run's pruning fetch succeeded; without it an
+	// origin/<branch> ref may be stale and due for deletion, so it proves nothing.
+	fetched bool
 }
 
 func newPruneCmd() *cobra.Command {
@@ -139,12 +142,15 @@ func plannedVerdict(r pruneResult, keepBranches bool) string {
 
 // newRepoCtx refreshes a source repo and collects the signals prune classifies
 // against. A failed fetch or an unreachable Gitea is reported and tolerated:
-// the git-only signals still work offline.
+// the signals that hold offline still work, and the rest are recorded as
+// unverified.
 func newRepoCtx(out io.Writer, prs prLister, srcDir string) (repoCtx, error) {
 	ctx := repoCtx{srcDir: srcDir, prs: map[string]agent.PullRequest{}}
 	repo := filepath.Base(srcDir)
 	if err := agent.GitFetchPrune(srcDir, "origin", credentialHelperArgs()...); err != nil {
-		_, _ = fmt.Fprintf(out, "warn: fetch %s: %v (using local refs)\n", repo, err)
+		_, _ = fmt.Fprintf(out, "warn: fetch %s: %v (remote state unverified)\n", repo, err)
+	} else {
+		ctx.fetched = true
 	}
 	def, err := agent.GitRemoteDefaultBranch(srcDir, "origin")
 	if err != nil {
@@ -223,7 +229,8 @@ func headContainedIn(dir, ref string) bool {
 // provably-upstream work loses its branch too, and anything unproven keeps its
 // branch so no commits become unreachable. A PR's state alone never authorises
 // deleting a branch — git must confirm HEAD is contained in what merged or in
-// what origin still holds.
+// what origin still holds, and origin's refs only count when this run's pruning
+// fetch refreshed them.
 func classify(wt managedWt, ctx repoCtx) (pruneResult, error) {
 	res := pruneResult{wt: wt}
 	dirty, err := agent.GitIsDirty(wt.path)
@@ -261,18 +268,22 @@ func classify(wt managedWt, ctx repoCtx) (pruneResult, error) {
 	}
 
 	remote := "origin/" + wt.branch
-	onOrigin := hasPR && agent.GitRemoteBranchExists(ctx.srcDir, "origin", wt.branch)
+	onOrigin := hasPR && ctx.fetched && agent.GitRemoteBranchExists(ctx.srcDir, "origin", wt.branch)
 	switch {
 	case hasPR && pr.Merged && headContainedIn(wt.path, pr.Head.Sha):
 		res.verdict, res.reason, res.proven = verdictRemoveBranch, fmt.Sprintf("PR merged #%d, HEAD contained in the merged head", pr.Number), true
 	case hasPR && pr.Merged && onOrigin && headContainedIn(wt.path, remote):
 		res.verdict, res.reason, res.proven = verdictRemoveBranch, fmt.Sprintf("PR merged #%d, HEAD contained in %s", pr.Number, remote), true
+	case hasPR && pr.Merged && !ctx.fetched:
+		res.verdict, res.reason = verdictRemove, fmt.Sprintf("PR merged #%d, fetch failed so %s is unverified", pr.Number, remote)
 	case hasPR && pr.Merged:
 		res.verdict, res.reason = verdictRemove, fmt.Sprintf("PR merged #%d, local commits not in the merged head", pr.Number)
 	case hasPR && onOrigin && headContainedIn(wt.path, remote):
 		res.verdict, res.reason, res.proven = verdictRemoveBranch, fmt.Sprintf("PR closed #%d, HEAD contained in %s", pr.Number, remote), true
 	case hasPR && onOrigin:
 		res.verdict, res.reason = verdictRemove, fmt.Sprintf("PR closed #%d, local commits not on %s", pr.Number, remote)
+	case hasPR && !ctx.fetched:
+		res.verdict, res.reason = verdictRemove, fmt.Sprintf("PR closed #%d, fetch failed so %s is unverified", pr.Number, remote)
 	case hasPR:
 		res.verdict, res.reason = verdictRemove, fmt.Sprintf("PR closed #%d, branch gone", pr.Number)
 	case ctx.prsKnown: