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/gitea.go

@@ -137,10 +137,63 @@ type PullRequest struct {
 	Mergeable bool   `json:"mergeable"`
 	HTMLURL   string `json:"html_url"`
 	Head      struct {
-		Sha string `json:"sha"`
+		Sha   string `json:"sha"`
+		Ref   string `json:"ref"`
+		Label string `json:"label"`
 	} `json:"head"`
 }
 
+// prPageSize is the per-page limit for the pulls listing; maxPRPages caps how
+// far back a listing walks.
+const (
+	prPageSize = 50
+	maxPRPages = 20
+)
+
+// ListPRs lists a repo's pull requests in the given state ("open", "closed" or
+// "all"), following pagination.
+func (c *GiteaClient) ListPRs(repoPath, state string) ([]PullRequest, error) {
+	if state == "" {
+		state = "all"
+	}
+	var all []PullRequest
+	for page := 1; page <= maxPRPages; page++ {
+		var batch []PullRequest
+		path := fmt.Sprintf("/api/v1/repos/%s/pulls?state=%s&limit=%d&page=%d", repoPath, state, prPageSize, page)
+		if err := c.do(http.MethodGet, path, nil, &batch); err != nil {
+			return nil, err
+		}
+		all = append(all, batch...)
+		if len(batch) < prPageSize {
+			break
+		}
+	}
+	return all, nil
+}
+
+// PRHeadBranch returns the branch a PR was opened from. Gitea rewrites head.ref
+// to "refs/pull/<n>/head" once the branch is deleted (which merging does), so
+// head.label — which keeps the original name — is authoritative.
+func PRHeadBranch(pr PullRequest) string {
+	if label := pr.Head.Label; label != "" && !strings.HasPrefix(label, "refs/pull/") {
+		// Cross-repo PRs label as "<owner>:<branch>".
+		if _, branch, ok := strings.Cut(label, ":"); ok {
+			return branch
+		}
+		return label
+	}
+	ref := pr.Head.Ref
+	if strings.HasPrefix(ref, "refs/pull/") {
+		return ""
+	}
+	return strings.TrimPrefix(ref, "refs/heads/")
+}
+
+// IsOpen reports whether a PR is still open (not merged, not closed).
+func (pr PullRequest) IsOpen() bool {
+	return pr.State == "open" && !pr.Merged
+}
+
 // CreatePROptions are the fields for opening a PR.
 type CreatePROptions struct {
 	Base  string `json:"base"`