add agentws prune

Agents leave their managed worktrees behind, and `agentws rm` takes one path at
a time with no idea whether a branch's work is safely upstream, so clearing an
accumulation by hand risks destroying unmerged commits.

- classify every managed worktree: dirty, PR open, upstream, or unproven
- remove only what is safe; delete the local branch only when work is upstream
- prove "upstream" with merge-base and git cherry, so squash merges count
- match a PR by head.label, which survives the branch deletion a merge does
- dry run by default; --yes applies, --keep-branches spares every branch
- read the Gitea path from origin's URL rather than assuming the owner

internal/agent/client_test.go

@@ -3,6 +3,7 @@ package agent
 import (
 	"encoding/json"
 	"errors"
+	"fmt"
 	"io"
 	"net/http"
 	"net/http/httptest"
@@ -219,6 +220,76 @@ func TestFetchStateFailsOnNon404StatusError(t *testing.T) {
 	}
 }
 
+// Gitea rewrites head.ref to "refs/pull/<n>/head" once the PR's branch is
+// deleted, which merging does in these repos. Matching a branch against
+// head.ref alone therefore finds nothing for every merged PR; head.label keeps
+// the original name.
+func TestPRHeadBranch(t *testing.T) {
+	tests := []struct {
+		name       string
+		ref, label string
+		want       string
+	}{
+		{"merged, branch deleted", "refs/pull/12/head", "benvin/merged", "benvin/merged"},
+		{"open PR", "benvin/open", "benvin/open", "benvin/open"},
+		{"fully qualified ref", "refs/heads/benvin/x", "", "benvin/x"},
+		{"no label falls back to ref", "benvin/y", "", "benvin/y"},
+		{"cross-repo label", "benvin/z", "someone:benvin/z", "benvin/z"},
+		{"nothing usable", "refs/pull/12/head", "", ""},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var pr PullRequest
+			pr.Head.Ref = tt.ref
+			pr.Head.Label = tt.label
+			if got := PRHeadBranch(pr); got != tt.want {
+				t.Errorf("PRHeadBranch(ref=%q,label=%q) = %q, want %q", tt.ref, tt.label, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestListPRsPaginates(t *testing.T) {
+	var pages []string
+	mux := http.NewServeMux()
+	mux.HandleFunc("/api/v1/repos/unkin/repo/pulls", func(w http.ResponseWriter, r *http.Request) {
+		q := r.URL.Query()
+		pages = append(pages, q.Get("page"))
+		if q.Get("state") != "all" {
+			t.Errorf("state = %q, want all", q.Get("state"))
+		}
+		if q.Get("page") == "1" {
+			full := make([]string, 0, prPageSize)
+			for i := 0; i < prPageSize; i++ {
+				full = append(full, fmt.Sprintf(`{"number":%d,"state":"closed","merged":true,"head":{"ref":"refs/pull/%d/head","label":"benvin/b%d"}}`, i+1, i+1, i+1))
+			}
+			_, _ = io.WriteString(w, "["+strings.Join(full, ",")+"]")
+			return
+		}
+		_, _ = io.WriteString(w, `[{"number":99,"state":"open","head":{"ref":"benvin/last","label":"benvin/last"}}]`)
+	})
+	srv := httptest.NewServer(mux)
+	defer srv.Close()
+
+	c := &GiteaClient{BaseURL: srv.URL, HTTP: srv.Client()}
+	prs, err := c.ListPRs("unkin/repo", "all")
+	if err != nil {
+		t.Fatalf("ListPRs: %v", err)
+	}
+	if len(prs) != prPageSize+1 {
+		t.Fatalf("got %d PRs, want %d", len(prs), prPageSize+1)
+	}
+	if len(pages) != 2 || pages[0] != "1" || pages[1] != "2" {
+		t.Errorf("pages requested = %v, want [1 2]", pages)
+	}
+	if got := PRHeadBranch(prs[0]); got != "benvin/b1" {
+		t.Errorf("first PR head branch = %q, want benvin/b1", got)
+	}
+	if !prs[len(prs)-1].IsOpen() {
+		t.Error("last PR should be open")
+	}
+}
+
 func TestGiteaAPIError(t *testing.T) {
 	mux := http.NewServeMux()
 	mux.HandleFunc("/api/v1/repos/unkin/repo/pulls", func(w http.ResponseWriter, r *http.Request) {