feat(pulls): add edit subcommand for pull requests (#944)

## Summary

- Add `tea pr edit` subcommand to support editing pull request properties (description, title, milestone, deadline, assignees, labels, reviewers)
- Add `--add-reviewers` / `--remove-reviewers` flags for managing PR reviewers via `CreateReviewRequests` / `DeleteReviewRequests` API
- Extract shared helpers (`ResolveLabelOpts`, `ApplyLabelChanges`, `ApplyReviewerChanges`, `ResolveMilestoneID`) into `modules/task/labels.go` to reduce duplication between issue and PR editing
- Refactor existing `EditIssue` to use the same shared helpers
- Wrap original error in `ResolveMilestoneID` to preserve underlying error context

## Usage

```bash
# Edit PR description
tea pr edit 1 --description "new description"

# Edit PR title
tea pr edit 1 --title "new title"

# Edit multiple fields
tea pr edit 1 --title "new title" --description "new desc" --add-labels "bug"

# Edit multiple PRs
tea pr edit 1 2 3 --add-assignees "user1"

# Add reviewers
tea pr edit 1 --add-reviewers "user1,user2"

# Remove reviewers
tea pr edit 1 --remove-reviewers "user1"
```

## Test plan

- [x] `go build .` succeeds
- [x] `go test ./...` passes
- [x] `make clean && make vet && make lint && make fmt-check && make docs-check && make build` all pass
- [x] `tea pr edit <idx> --description "test"` updates PR description on a Gitea instance
- [x] `tea pr edit <idx> --title "test"` updates PR title
- [x] `tea pr edit <idx> --add-labels "bug"` adds label
- [x] `tea pr edit <idx> --add-reviewers "user"` requests review
- [x] `tea pr edit <idx> --remove-reviewers "user"` removes reviewer
- [x] Existing `tea issues edit` still works correctly after refactor

Reviewed-on: https://gitea.com/gitea/tea/pulls/944
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: appleboy <appleboy.tw@gmail.com>
Co-committed-by: appleboy <appleboy.tw@gmail.com>

modules/task/labels.go

@@ -4,6 +4,8 @@
 package task
 
 import (
+	"fmt"
+
 	"code.gitea.io/sdk/gitea"
 	"code.gitea.io/tea/modules/utils"
 )
@@ -24,3 +26,68 @@ func ResolveLabelNames(client *gitea.Client, owner, repo string, labelNames []st
 	}
 	return labelIDs, nil
 }
+
+// ResolveLabelOpts resolves label names to IssueLabelsOption. Returns nil if names is empty.
+func ResolveLabelOpts(client *gitea.Client, owner, repo string, names []string) (*gitea.IssueLabelsOption, error) {
+	if len(names) == 0 {
+		return nil, nil
+	}
+	ids, err := ResolveLabelNames(client, owner, repo, names)
+	if err != nil {
+		return nil, err
+	}
+	return &gitea.IssueLabelsOption{Labels: ids}, nil
+}
+
+// ApplyLabelChanges adds and removes labels on an issue or pull request.
+func ApplyLabelChanges(client *gitea.Client, owner, repo string, index int64, add, rm *gitea.IssueLabelsOption) error {
+	if rm != nil {
+		// NOTE: as of 1.17, there is no API to remove multiple labels at once.
+		for _, id := range rm.Labels {
+			_, err := client.DeleteIssueLabel(owner, repo, index, id)
+			if err != nil {
+				return fmt.Errorf("could not remove labels: %s", err)
+			}
+		}
+	}
+	if add != nil {
+		_, _, err := client.AddIssueLabels(owner, repo, index, *add)
+		if err != nil {
+			return fmt.Errorf("could not add labels: %s", err)
+		}
+	}
+	return nil
+}
+
+// ApplyReviewerChanges adds and removes reviewers on a pull request.
+func ApplyReviewerChanges(client *gitea.Client, owner, repo string, index int64, add, rm []string) error {
+	if len(rm) != 0 {
+		_, err := client.DeleteReviewRequests(owner, repo, index, gitea.PullReviewRequestOptions{
+			Reviewers: rm,
+		})
+		if err != nil {
+			return fmt.Errorf("could not remove reviewers: %w", err)
+		}
+	}
+	if len(add) != 0 {
+		_, err := client.CreateReviewRequests(owner, repo, index, gitea.PullReviewRequestOptions{
+			Reviewers: add,
+		})
+		if err != nil {
+			return fmt.Errorf("could not add reviewers: %w", err)
+		}
+	}
+	return nil
+}
+
+// ResolveMilestoneID resolves a milestone name to its ID. Returns 0 for empty name.
+func ResolveMilestoneID(client *gitea.Client, owner, repo, name string) (int64, error) {
+	if name == "" {
+		return 0, nil
+	}
+	ms, _, err := client.GetMilestoneByName(owner, repo, name)
+	if err != nil {
+		return 0, fmt.Errorf("could not resolve milestone '%s': %w", name, err)
+	}
+	return ms.ID, nil
+}