core: track agent and model used in each session

Store the active agent and model in the session table so users can see which configuration was used when browsing session history. This helps identify sessions that used specific agents or models for easier filtering and organization.

Author: thdxr

packages/opencode/migration/20260428004200_add_session_path/snapshot.json

@@ -0,0 +1,2 @@
+ALTER TABLE `session` ADD `agent` text;--> statement-breakpoint
+ALTER TABLE `session` ADD `model` text;