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/querier.go

@@ -490,6 +490,7 @@ type sqlcQuerier interface {
 	GetUserActivityInsights(ctx context.Context, arg GetUserActivityInsightsParams) ([]GetUserActivityInsightsRow, error)
 	GetUserByEmailOrUsername(ctx context.Context, arg GetUserByEmailOrUsernameParams) (User, error)
 	GetUserByID(ctx context.Context, id uuid.UUID) (User, error)
+	GetUserChatCustomPrompt(ctx context.Context, userID uuid.UUID) (string, error)
 	GetUserCount(ctx context.Context, includeSystem bool) (int64, error)
 	// GetUserLatencyInsights returns the median and 95th percentile connection
 	// latency that users have experienced. The result can be filtered on
@@ -789,6 +790,7 @@ type sqlcQuerier interface {
 	UpdateTemplateVersionFlagsByJobID(ctx context.Context, arg UpdateTemplateVersionFlagsByJobIDParams) error
 	UpdateTemplateWorkspacesLastUsedAt(ctx context.Context, arg UpdateTemplateWorkspacesLastUsedAtParams) error
 	UpdateUsageEventsPostPublish(ctx context.Context, arg UpdateUsageEventsPostPublishParams) error
+	UpdateUserChatCustomPrompt(ctx context.Context, arg UpdateUserChatCustomPromptParams) (UserConfig, error)
 	UpdateUserDeletedByID(ctx context.Context, id uuid.UUID) error
 	UpdateUserGithubComUserID(ctx context.Context, arg UpdateUserGithubComUserIDParams) error
 	UpdateUserHashedOneTimePasscode(ctx context.Context, arg UpdateUserHashedOneTimePasscodeParams) error