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.

coderd/database/queries.sql.go

@@ -18744,6 +18744,23 @@ func (q *sqlQuerier) GetUserByID(ctx context.Context, id uuid.UUID) (User, error
 	return i, err
 }
 
+const getUserChatCustomPrompt = `-- name: GetUserChatCustomPrompt :one
+SELECT
+	value as chat_custom_prompt
+FROM
+	user_configs
+WHERE
+	user_id = $1
+	AND key = 'chat_custom_prompt'
+`
+
+func (q *sqlQuerier) GetUserChatCustomPrompt(ctx context.Context, userID uuid.UUID) (string, error) {
+	row := q.db.QueryRowContext(ctx, getUserChatCustomPrompt, userID)
+	var chat_custom_prompt string
+	err := row.Scan(&chat_custom_prompt)
+	return chat_custom_prompt, err
+}
+
 const getUserCount = `-- name: GetUserCount :one
 SELECT
 	COUNT(*)
@@ -19191,6 +19208,33 @@ func (q *sqlQuerier) UpdateInactiveUsersToDormant(ctx context.Context, arg Updat
 	return items, nil
 }
 
+const updateUserChatCustomPrompt = `-- name: UpdateUserChatCustomPrompt :one
+INSERT INTO
+	user_configs (user_id, key, value)
+VALUES
+	($1, 'chat_custom_prompt', $2)
+ON CONFLICT
+	ON CONSTRAINT user_configs_pkey
+DO UPDATE
+SET
+	value = $2
+WHERE user_configs.user_id = $1
+	AND user_configs.key = 'chat_custom_prompt'
+RETURNING user_id, key, value
+`
+
+type UpdateUserChatCustomPromptParams struct {
+	UserID           uuid.UUID `db:"user_id" json:"user_id"`
+	ChatCustomPrompt string    `db:"chat_custom_prompt" json:"chat_custom_prompt"`
+}
+
+func (q *sqlQuerier) UpdateUserChatCustomPrompt(ctx context.Context, arg UpdateUserChatCustomPromptParams) (UserConfig, error) {
+	row := q.db.QueryRowContext(ctx, updateUserChatCustomPrompt, arg.UserID, arg.ChatCustomPrompt)
+	var i UserConfig
+	err := row.Scan(&i.UserID, &i.Key, &i.Value)
+	return i, err
+}
+
 const updateUserDeletedByID = `-- name: UpdateUserDeletedByID :exec
 UPDATE
 	users