feat(core): add session metadata support

Persist session metadata in the session store and expose it across the session APIs and SDK so clients can round-trip metadata during create, get, update, list, and fork operations.

Add a JSON metadata column to the session table, thread metadata through the session model/projectors, and normalize empty metadata responses to {}.

Support full metadata replacement on session update, add copyMetadata to session forks with default copy behavior, regenerate the session SDK surface, and cover the behavior with server/session tests plus a migration for existing databases.

Author: shantur

packages/opencode/migration/20260417072500_session-metadata/migration.sql

@@ -0,0 +1 @@
+ALTER TABLE `session` ADD `metadata` text;