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/gitprovider.go

@@ -74,6 +74,8 @@ type PRStatus struct {
 	Draft bool
 	// HeadSHA is the SHA of the head commit.
 	HeadSHA string
+	// HeadBranch is the name of the branch containing the PR changes.
+	HeadBranch string
 	// DiffStats summarizes additions/deletions/files changed.
 	DiffStats DiffStats
 	// ChangesRequested is a convenience boolean: true if any