feat: add per-chat system prompt option (#24053)

Adds a `system_prompt` field to `CreateChatRequest` that allows API
consumers to provide custom instructions when creating a chat. The
per-chat prompt is stored as a separate system message (`role=system`,
`visibility=model`) in the `chat_messages` table, inserted between the
deployment system prompt and the workspace awareness message.

Also moves deployment system prompt resolution from the HTTP handler
(`resolvedChatSystemPrompt`) into `chatd.CreateChat` where it belongs.
The handler no longer assembles system prompts —
`CreateOptions.SystemPrompt` is now purely the per-chat user prompt, and
the deployment prompt is resolved internally by chatd.

No database schema changes required.

**Message insertion order:**
1. Deployment system prompt (resolved by chatd, existing)
2. Per-chat user system prompt (new, from `CreateOptions.SystemPrompt`)
3. Workspace awareness (existing)
4. Initial user message (existing)

🤖 Generated with [Coder Agents](https://coder.com/agents)

codersdk/chats.go

@@ -345,6 +345,7 @@ type ChatInputPart struct {
 // CreateChatRequest is the request to create a new chat.
 type CreateChatRequest struct {
 	Content       []ChatInputPart   `json:"content"`
+	SystemPrompt  string            `json:"system_prompt,omitempty"`
 	WorkspaceID   *uuid.UUID        `json:"workspace_id,omitempty" format:"uuid"`
 	ModelConfigID *uuid.UUID        `json:"model_config_id,omitempty" format:"uuid"`
 	MCPServerIDs  []uuid.UUID       `json:"mcp_server_ids,omitempty" format:"uuid"`