fix(core): copy the context checkpoint when forking a session

Copied fork messages keep their parent seqs, but a fresh checkpoint's
baseline_seq starts near zero, so nothing folded: forks received a new
full baseline plus every stale context-update message from the parent,
and copied compaction rows triggered a spurious rebaseline. Inheriting
the parent row keeps the fold horizon consistent with the copied
transcript.

Also asserts the checkpoint reset on committed reverts and deletes the
unused SessionStore.runnerContext / SessionHistory.loadForRunner
surface.

packages/core/test/session-projector.test.ts

@@ -19,7 +19,12 @@ import { SessionMessageUpdater } from "@opencode-ai/core/session/message-updater
 import { SessionProjector } from "@opencode-ai/core/session/projector"
 import { SessionExecution } from "@opencode-ai/core/session/execution"
 import { SessionInput } from "@opencode-ai/core/session/input"
-import { SessionInputTable, SessionMessageTable, SessionTable } from "@opencode-ai/core/session/sql"
+import {
+  SessionContextCheckpointTable,
+  SessionInputTable,
+  SessionMessageTable,
+  SessionTable,
+} from "@opencode-ai/core/session/sql"
 import { testEffect } from "./lib/effect"
 import { Snapshot } from "@opencode-ai/core/snapshot"
 
@@ -67,6 +72,10 @@ describe("SessionProjector", () => {
         .insert(SessionMessageTable)
         .values([assistantRow(boundary, 1), assistantRow(SessionMessage.ID.make("msg_later"), 2)])
         .run()
+      yield* db
+        .insert(SessionContextCheckpointTable)
+        .values({ session_id: sessionID, baseline: "baseline", snapshot: {}, baseline_seq: 0 })
+        .run()
       const events = yield* EventV2.Service
       yield* events.publish(SessionEvent.RevertEvent.Staged, {
         sessionID,
@@ -93,6 +102,8 @@ describe("SessionProjector", () => {
       expect(
         (yield* db.select({ id: SessionMessageTable.id }).from(SessionMessageTable).all()).map((row) => row.id),
       ).toEqual([boundary])
+      // A committed revert resets the context checkpoint so the next turn re-initializes.
+      expect(yield* db.select().from(SessionContextCheckpointTable).get().pipe(Effect.orDie)).toBeUndefined()
     }),
   )