feat(core): persist v2 session context epochs

packages/core/migration/20260604180746_add_session_context_epoch/migration.sql

@@ -0,0 +1,7 @@
+CREATE TABLE `session_context_epoch` (
+	`session_id` text PRIMARY KEY,
+	`baseline` text NOT NULL,
+	`checkpoint` text NOT NULL,
+	`baseline_seq` integer NOT NULL,
+	CONSTRAINT `fk_session_context_epoch_session_id_session_id_fk` FOREIGN KEY (`session_id`) REFERENCES `session`(`id`) ON DELETE CASCADE
+);

packages/core/migration/20260604181329_add_session_context_updates/migration.sql

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+CREATE TABLE `session_context_message` (
+	`session_id` text NOT NULL,
+	`seq` integer NOT NULL,
+	`parts` text NOT NULL,
+	CONSTRAINT `session_context_message_pk` PRIMARY KEY(`session_id`, `seq`),
+	CONSTRAINT `fk_session_context_message_session_id_session_id_fk` FOREIGN KEY (`session_id`) REFERENCES `session`(`id`) ON DELETE CASCADE
+);
+--> statement-breakpoint
+ALTER TABLE `session_context_epoch` ADD `revision` integer DEFAULT 0 NOT NULL;--> statement-breakpoint
+CREATE INDEX `session_context_message_session_seq_idx` ON `session_context_message` (`session_id`,`seq`);

packages/core/migration/20260604181706_add_session_context_replacement/migration.sql

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+ALTER TABLE `session_context_epoch` ADD `replacement_pending` integer DEFAULT false NOT NULL;

packages/core/migration/20260604181807_add_session_context_replacement_sequence/migration.sql

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+ALTER TABLE `session_context_epoch` ADD `replacement_seq` integer;