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/queries/chats.sql

@@ -136,7 +136,6 @@ ORDER BY
 INSERT INTO chats (
     owner_id,
     workspace_id,
-    workspace_agent_id,
     parent_chat_id,
     root_chat_id,
     last_model_config_id,
@@ -144,7 +143,6 @@ INSERT INTO chats (
 ) VALUES (
     @owner_id::uuid,
     sqlc.narg('workspace_id')::uuid,
-    sqlc.narg('workspace_agent_id')::uuid,
     sqlc.narg('parent_chat_id')::uuid,
     sqlc.narg('root_chat_id')::uuid,
     @last_model_config_id::uuid,
@@ -222,7 +220,6 @@ UPDATE
     chats
 SET
     workspace_id = sqlc.narg('workspace_id')::uuid,
-    workspace_agent_id = sqlc.narg('workspace_agent_id')::uuid,
     updated_at = NOW()
 WHERE
     id = @id::uuid