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/database/models.go

@@ -3889,7 +3889,6 @@ type Chat struct {
 	ID                uuid.UUID      `db:"id" json:"id"`
 	OwnerID           uuid.UUID      `db:"owner_id" json:"owner_id"`
 	WorkspaceID       uuid.NullUUID  `db:"workspace_id" json:"workspace_id"`
-	WorkspaceAgentID  uuid.NullUUID  `db:"workspace_agent_id" json:"workspace_agent_id"`
 	Title             string         `db:"title" json:"title"`
 	Status            ChatStatus     `db:"status" json:"status"`
 	WorkerID          uuid.NullUUID  `db:"worker_id" json:"worker_id"`