chore(db): remove workspace_agent_id from chats table (#22442)

## Summary

Remove the `workspace_agent_id` column from the `chats` table and
dynamically look up the first workspace agent instead.

## Problem

When a workspace is stopped and restarted, the workspace agent gets a
new ID. The `workspace_agent_id` stored on the chat at creation time
becomes stale, making the agent unreachable. This caused chats to break
after workspace restarts.

## Solution

Instead of persisting the agent ID, dynamically look up the first agent
from the workspace's latest build via
`GetWorkspaceAgentsInLatestBuildByWorkspaceID` whenever an agent
connection is needed. The `workspace_id` on the chat remains stable
across restarts.

This behavior may be refined later (e.g., agent selection heuristics),
but picking the first agent resolves the immediate breakage.

## Changes

- **Migration 000425**: Drop `workspace_agent_id` column from `chats`
- **SQL queries**: Remove `workspace_agent_id` from `InsertChat` and
`UpdateChatWorkspace`
- **chatd.go**: `getWorkspaceConn` and `resolveInstructions` now look up
agents dynamically from workspace ID
- **chatd.go**: Remove `refreshChatWorkspaceSnapshot` (no longer needed)
- **createworkspace.go**: Stop persisting agent ID when associating
workspace with chat
- **subagent.go**: Stop passing agent ID to child chats
- **SDK/frontend**: Remove `WorkspaceAgentID` / `workspace_agent_id`
from Chat type

---------

Co-authored-by: Kyle Carberry <kylecarbs@gmail.com>

coderd/chats.go

@@ -258,7 +258,6 @@ func (api *API) postChats(rw http.ResponseWriter, r *http.Request) {
 	chat, err := api.chatDaemon.CreateChat(ctx, chatd.CreateOptions{
 		OwnerID:            apiKey.UserID,
 		WorkspaceID:        workspaceSelection.WorkspaceID,
-		WorkspaceAgentID:   workspaceSelection.WorkspaceAgentID,
 		Title:              title,
 		ModelConfigID:      modelConfigID,
 		SystemPrompt:       defaultChatSystemPrompt(),
@@ -1854,8 +1853,7 @@ func parseGitHubPullRequestURL(raw string) (githubPullRequestRef, bool) {
 }
 
 type createChatWorkspaceSelection struct {
-	WorkspaceID      uuid.NullUUID
-	WorkspaceAgentID uuid.NullUUID
+	WorkspaceID uuid.NullUUID
 }
 
 func (api *API) validateCreateChatWorkspaceSelection(
@@ -1888,23 +1886,6 @@ func (api *API) validateCreateChatWorkspaceSelection(
 		Valid: true,
 	}
 
-	workspaceAgents, err := api.Database.GetWorkspaceAgentsInLatestBuildByWorkspaceID(
-		ctx,
-		workspace.ID,
-	)
-	if err != nil {
-		return selection, http.StatusInternalServerError, &codersdk.Response{
-			Message: "Failed to get workspace agents.",
-			Detail:  err.Error(),
-		}
-	}
-	if len(workspaceAgents) > 0 {
-		selection.WorkspaceAgentID = uuid.NullUUID{
-			UUID:  workspaceAgents[0].ID,
-			Valid: true,
-		}
-	}
-
 	return selection, 0, nil
 }
 
@@ -2081,9 +2062,6 @@ func convertChat(c database.Chat, diffStatus *database.ChatDiffStatus) codersdk.
 	if c.WorkspaceID.Valid {
 		chat.WorkspaceID = &c.WorkspaceID.UUID
 	}
-	if c.WorkspaceAgentID.Valid {
-		chat.WorkspaceAgentID = &c.WorkspaceAgentID.UUID
-	}
 	if diffStatus != nil {
 		convertedDiffStatus := convertChatDiffStatus(c.ID, diffStatus)
 		chat.DiffStatus = &convertedDiffStatus