fix(print): distinguish draft PRs from conflicting PRs (#1012)

## The bug

`tea pulls create` and `tea pulls edit` print `• **Conflicting files**` for any open PR that the server reports as `mergeable: false`. But Gitea's API returns `mergeable: false` for **draft PRs by design** — drafts cannot be merged regardless of conflict state. The current code conflates "not mergeable for any reason" with "has file conflicts."

### Reproduction (gitea.com 1.26.0+dev)

```
$ tea pulls create --base main --head my-branch --title "WIP: clean diff" --description "..."
  # #N WIP: clean diff (open)
  ...
  • **Conflicting files**           ← misleading
  • Maintainers are allowed to edit
```

The PR has no actual conflicts. The web UI shows it as draft + cleanly diffable. `tea pulls edit --title "WIP: ..."` shows the same misleading line.

API confirms the root signal:

```
$ curl /api/v1/repos/owner/repo/pulls/N
{ "draft": true, "mergeable": false, ... }
```

Strip the WIP prefix, and `mergeable` flips back to `true`. So `mergeable=false` here means "blocked because draft," not "conflicts."

## Fix

Distinguish the two reasons in `modules/print/pull.go`:

```go
switch {
case pr.Mergeable:
    out += "- No Conflicts\n"
case pr.Draft:
    out += "- Draft (not mergeable until marked ready)\n"
default:
    out += "- **Conflicting files**\n"
}
```

Real conflicts still fall through to the existing "Conflicting files" message.

## Behavior matrix

| PR state | API `mergeable` | Before | After |
|---|---|---|---|
| Open, clean, ready | true | No Conflicts | No Conflicts |
| Open, draft, clean | false | **Conflicting files** (wrong) | **Draft (not mergeable until marked ready)** |
| Open, conflicting | false | Conflicting files | Conflicting files |
| Closed/merged | — | (no line shown) | (no line shown) |

## Verification

Against `dinsmoor/tea-testing` PR #6 on gitea.com:

- Set title to `WIP: ...` → API `mergeable=false`, `draft=true` → tea prints `Draft (not mergeable until marked ready)` ✓
- Strip WIP → API `mergeable=true`, `draft=false` → tea prints `No Conflicts` ✓

---

This patch was authored interactively with an AI assistant, driven and reviewed by a human (Tyler / @dinsmoor) every step.

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

Reviewed-on: https://gitea.com/gitea/tea/pulls/1012
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Tyler <tyler@dinsmoor.us>
Co-committed-by: Tyler <tyler@dinsmoor.us>

modules/print/pull.go

@@ -59,9 +59,12 @@ func PullDetails(pr *gitea.PullRequest, reviews []*gitea.PullReview, ciStatus *g
 	}
 
 	if pr.State == gitea.StateOpen {
-		if pr.Mergeable {
+		switch {
+		case pr.Mergeable:
 			out += "- No Conflicts\n"
-		} else {
+		case pr.Draft:
+			out += "- Draft (not mergeable until marked ready)\n"
+		default:
 			out += "- **Conflicting files**\n"
 		}
 	}