feat: add head_branch to pull request diff status (#23076)

Adds the `head_branch` field (the source/feature branch name of a PR) to
the diff status pipeline. Previously only `base_branch` (target branch)
and the head commit SHA were captured from the GitHub API, but not the
head branch name itself.

## Changes

- **Migration 438**: Add `head_branch` nullable TEXT column to
`chat_diff_statuses`
- **gitprovider**: Parse `head.ref` from the GitHub API response
(alongside `head.sha`) and add `HeadBranch` to `PRStatus`
- **gitsync**: Wire `HeadBranch` through `refreshOne()` into the DB
upsert params
- **worker**: Map `HeadBranch` in `chatDiffStatusFromRow()`
- **coderd**: Convert `HeadBranch` in `convertChatDiffStatus()`
- **codersdk**: Expose as `head_branch` (`*string`, omitempty) in
`ChatDiffStatus` API response
- **Tests**: Updated `github_test.go` pull JSON fixtures and assertions

coderd/externalauth/gitprovider/github.go

@@ -269,6 +269,7 @@ func (g *githubProvider) FetchPullRequestStatus(
 		Commits      int32  `json:"commits"`
 		Head         struct {
 			SHA string `json:"sha"`
+			Ref string `json:"ref"`
 		} `json:"head"`
 		User struct {
 			Login     string `json:"login"`
@@ -310,10 +311,11 @@ func (g *githubProvider) FetchPullRequestStatus(
 	reviewInfo := summarizeReviews(reviews)
 
 	return &PRStatus{
-		Title:   pull.Title,
-		State:   state,
-		Draft:   pull.Draft,
-		HeadSHA: pull.Head.SHA,
+		Title:      pull.Title,
+		State:      state,
+		Draft:      pull.Draft,
+		HeadSHA:    pull.Head.SHA,
+		HeadBranch: pull.Head.Ref,
 		DiffStats: DiffStats{
 			Additions:    pull.Additions,
 			Deletions:    pull.Deletions,