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>

modules/utils/draft.go

@@ -0,0 +1,31 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package utils
+
+import "regexp"
+
+// draftPrefixRe matches the prefixes Gitea recognizes as marking a pull
+// request as a draft: a leading "WIP:" or "[WIP]" (case-insensitive),
+// followed by any whitespace.
+var draftPrefixRe = regexp.MustCompile(`(?i)^(wip:\s*|\[wip\]\s*)`)
+
+// HasDraftPrefix reports whether title already starts with a draft marker.
+func HasDraftPrefix(title string) bool {
+	return draftPrefixRe.MatchString(title)
+}
+
+// AddDraftPrefix returns title with a "WIP: " prefix, or title unchanged
+// if it already carries a recognized draft prefix.
+func AddDraftPrefix(title string) string {
+	if HasDraftPrefix(title) {
+		return title
+	}
+	return "WIP: " + title
+}
+
+// StripDraftPrefix returns title with any recognized leading draft prefix
+// removed, or title unchanged if no prefix is present.
+func StripDraftPrefix(title string) string {
+	return draftPrefixRe.ReplaceAllString(title, "")
+}