feat(pulls): add --draft to create and --draft/--ready to edit (#1008)

Closes #979. An alternative to the approach in #980 — this one is purely client-side title-mangling, no SDK changes needed.

Gitea already treats any PR with a `WIP:` or `[WIP]` title prefix (case-insensitive) as a draft. This patch wires three flags around that behavior:

- `tea pulls create --draft` — prepend `WIP: ` to the title at creation time
- `tea pulls edit --draft <idx>` — add `WIP: ` to an existing PR's title
- `tea pulls edit --ready <idx>` — strip any recognized draft prefix

All three are idempotent. `--draft` and `--ready` on edit are mutually exclusive. If the user also passes `--title` on edit, the toggle applies to the supplied title; otherwise the current title is fetched from the server first.

Why this approach over a server-payload-based one: Gitea's draft state is *defined* as the title-prefix convention (see the Gitea source for `HasWIPPrefix`). Modeling it server-side would either duplicate or fight that. A small string helper covers it without needing the SDK to add a `Draft` field.

Verified against `gitea.com` (1.26.0+dev) with a throwaway repo:
- create with `--draft` → server reports `draft: true` ✓
- `edit --ready` strips → `draft: false` ✓
- `edit --draft` adds back → `draft: true` ✓
- second `edit --draft` is idempotent ✓
- `edit --draft --ready` errors ✓

Unit tests for the prefix detection live in `modules/utils/draft_test.go`.

---

This patch was authored interactively with an AI assistant, driven and reviewed by a human (Tyler / @dinsmoor) every step. Reproduction, design decisions, and the choice not to follow #980's payload approach were mine — happy to discuss any of it.

*pull request created by Tyler's lovingly wrangled demon machine <3*

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-on: https://gitea.com/gitea/tea/pulls/1008
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Tyler <tyler@dinsmoor.us>
Co-committed-by: Tyler <tyler@dinsmoor.us>

cmd/pulls/create.go

@@ -13,6 +13,7 @@ import (
 	"gitea.dev/tea/modules/context"
 	"gitea.dev/tea/modules/interact"
 	"gitea.dev/tea/modules/task"
+	"gitea.dev/tea/modules/utils"
 )
 
 // CmdPullsCreate creates a pull request
@@ -46,6 +47,10 @@ var CmdPullsCreate = cli.Command{
 			Name:  "topic",
 			Usage: "Topic name for agit flow pull request",
 		},
+		&cli.BoolFlag{
+			Name:  "draft",
+			Usage: "Create as a draft (prepends \"WIP: \" to the title; Gitea treats WIP-prefixed PRs as drafts)",
+		},
 	}, flags.IssuePRCreateFlags...),
 }
 
@@ -81,6 +86,10 @@ func runPullsCreate(requestCtx stdctx.Context, cmd *cli.Command) error {
 		return err
 	}
 
+	if ctx.Bool("draft") {
+		opts.Title = utils.AddDraftPrefix(opts.Title)
+	}
+
 	if ctx.Bool("agit") {
 		return task.CreateAgitFlowPull(
 			requestCtx,