feat(chatd): add user-level custom prompt for agent chats (#22896)

Adds a user-level custom prompt to the database.

I'll be doing a follow-up for the UI, as we currently do not have
user-level settings (it's just admin). I'll also make it very obvious
for chats where there is a user-level prompt, but I don't know how yet.

codersdk/chats.go

@@ -211,6 +211,18 @@ type UpdateChatSystemPromptRequest struct {
 	SystemPrompt string `json:"system_prompt"`
 }
 
+// UserChatCustomPromptResponse is the response for getting a user's
+// custom chat prompt.
+type UserChatCustomPromptResponse struct {
+	CustomPrompt string `json:"custom_prompt"`
+}
+
+// UpdateUserChatCustomPromptRequest is the request to update a user's
+// custom chat prompt.
+type UpdateUserChatCustomPromptRequest struct {
+	CustomPrompt string `json:"custom_prompt"`
+}
+
 // ChatProviderConfigSource describes how a provider entry is sourced.
 type ChatProviderConfigSource string
 
@@ -725,6 +737,34 @@ func (c *Client) UpdateChatSystemPrompt(ctx context.Context, req UpdateChatSyste
 	return nil
 }
 
+// GetUserChatCustomPrompt fetches the user's custom chat prompt.
+func (c *Client) GetUserChatCustomPrompt(ctx context.Context) (UserChatCustomPromptResponse, error) {
+	res, err := c.Request(ctx, http.MethodGet, "/api/experimental/chats/config/user-prompt", nil)
+	if err != nil {
+		return UserChatCustomPromptResponse{}, err
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		return UserChatCustomPromptResponse{}, ReadBodyAsError(res)
+	}
+	var resp UserChatCustomPromptResponse
+	return resp, json.NewDecoder(res.Body).Decode(&resp)
+}
+
+// UpdateUserChatCustomPrompt updates the user's custom chat prompt.
+func (c *Client) UpdateUserChatCustomPrompt(ctx context.Context, req UpdateUserChatCustomPromptRequest) (UserChatCustomPromptResponse, error) {
+	res, err := c.Request(ctx, http.MethodPut, "/api/experimental/chats/config/user-prompt", req)
+	if err != nil {
+		return UserChatCustomPromptResponse{}, err
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		return UserChatCustomPromptResponse{}, ReadBodyAsError(res)
+	}
+	var resp UserChatCustomPromptResponse
+	return resp, json.NewDecoder(res.Body).Decode(&resp)
+}
+
 // CreateChat creates a new chat.
 func (c *Client) CreateChat(ctx context.Context, req CreateChatRequest) (Chat, error) {
 	res, err := c.Request(ctx, http.MethodPost, "/api/experimental/chats", req)