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>

cmd/pulls.go

@@ -72,6 +72,7 @@ var CmdPulls = cli.Command{
 		&pulls.CmdPullsCreate,
 		&pulls.CmdPullsClose,
 		&pulls.CmdPullsReopen,
+		&pulls.CmdPullsEdit,
 		&pulls.CmdPullsReview,
 		&pulls.CmdPullsApprove,
 		&pulls.CmdPullsReject,