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/chatd/chattool/createworkspace.go

@@ -191,10 +191,6 @@ func CreateWorkspace(options CreateWorkspaceOptions) fantasy.AgentTool {
 						UUID:  workspace.ID,
 						Valid: true,
 					},
-					WorkspaceAgentID: uuid.NullUUID{
-						UUID:  workspaceAgentID,
-						Valid: workspaceAgentID != uuid.Nil,
-					},
 				})
 			}
 
@@ -281,14 +277,15 @@ func checkExistingWorkspace(
 
 	case database.ProvisionerJobStatusSucceeded:
 		// Build succeeded — check if agent is reachable.
-		if chat.WorkspaceAgentID.Valid && agentConnFn != nil {
+		agents, agentsErr := db.GetWorkspaceAgentsInLatestBuildByWorkspaceID(ctx, ws.ID)
+		if agentsErr == nil && len(agents) > 0 && agentConnFn != nil {
 			pingCtx, cancel := context.WithTimeout(
 				ctx, agentPingTimeout,
 			)
 			defer cancel()
 
 			conn, release, connErr := agentConnFn(
-				pingCtx, chat.WorkspaceAgentID.UUID,
+				pingCtx, agents[0].ID,
 			)
 			if connErr == nil {
 				release()