feat(agents): add PR title, draft, and status icons to sidebar (#22952)

Adds `pull_request_title` and `pull_request_draft` to the chat diff
status pipeline (DB → provider → SDK → frontend). The GitHub provider
now fetches the PR title alongside existing status fields.

The agents sidebar now displays PR-state-aware icons for chats that have
a linked pull request (when the chat is in waiting/completed state):
- **Open PR**: `GitPullRequestArrow` (green)
- **Draft PR**: `GitPullRequestDraft` (gray)
- **Merged PR**: `GitMerge` (purple)
- **Closed PR**: `GitPullRequestClosed` (red)

Running/pending/paused/error chats keep their existing activity icons
(spinner, pause, error triangle).

### Changes

**Database migration** (`000432`): Adds `pull_request_title TEXT` and
`pull_request_draft BOOLEAN` columns to `chat_diff_statuses`.

**Backend pipeline**:
- `gitprovider.PRStatus` gains a `Title` field
- GitHub provider decodes the `title` from the API response
- `gitsync` and `coderd/chats.go` pass title + draft through to the DB
upsert
- `codersdk.ChatDiffStatus` exposes both new fields in the API response

**Frontend** (`AgentsSidebar.tsx`): New `getPRIconConfig()` function
resolves the appropriate Lucide git icon based on `pull_request_state`
and `pull_request_draft`. Only applies when the chat is in a terminal
state (waiting/completed).

**Real-time sync**: No changes needed — the existing
`diff_status_change` pubsub event already propagates the full
`ChatDiffStatus` including the new fields.

coderd/externalauth/gitprovider/gitprovider.go

@@ -66,6 +66,8 @@ type DiffStats struct {
 // PRStatus is the complete status of a pull/merge request.
 // This is the universal return type that all providers populate.
 type PRStatus struct {
+	// Title is the PR's title/subject line.
+	Title string
 	// State is the PR's lifecycle state.
 	State PRState
 	// Draft indicates the PR is marked as draft/WIP.