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>

docs/CLI.md

@@ -375,6 +375,8 @@ Create a pull-request
 
 **--description, -d**="": 
 
+**--draft**: Create as a draft (prepends "WIP: " to the title; Gitea treats WIP-prefixed PRs as drafts)
+
 **--head**="": Branch name of the PR source (default is current one). To specify a different head repo, use <user>:<branch>
 
 **--labels, -L**="": Comma-separated list of labels to assign
@@ -431,10 +433,14 @@ Edit one or more pull requests
 
 **--description, -d**="": 
 
+**--draft**: Mark as draft by prepending "WIP: " to the title (idempotent)
+
 **--login, -l**="": Use a different Gitea Login. Optional
 
 **--milestone, -m**="": Milestone to assign
 
+**--ready**: Mark as ready for review by stripping any leading "WIP: " or "[WIP]" prefix
+
 **--referenced-version, -v**="": commit-hash or tag name to assign
 
 **--remote, -R**="": Discover Gitea login from remote. Optional