refactor(session): drop the unused predicate helper and tighten the docs

`isInheritedSeq` had no production caller — only its own tests — so it was
a public core export shaped by nothing. A bracket owner reads the boundary
positionally; the helper belongs with the compaction seam, where a real
consumer decides its signature.

Also condense the `session/inherited` and `lastActivityTime` docs.

packages/core/session/src/index.ts

@@ -23,7 +23,7 @@ export * from './types.ts'
 export type { AssistantMessage, ToolResultMessage, UserMessage } from '@deepseek-ai/dsh-llm'
 export { isJsonValue, snapshotJsonValue } from './json.ts'
 export type { JsonValue } from './json.ts'
-export { interruptedTurnClosers, isInheritedSeq, lastActivityTime, TOOL_NOT_STARTED, TOOL_OUTCOME_UNKNOWN } from './repair.ts'
+export { interruptedTurnClosers, lastActivityTime, TOOL_NOT_STARTED, TOOL_OUTCOME_UNKNOWN } from './repair.ts'
 export { decodeStorageRecord, packChunkRuns } from './chunk-rows.ts'
 export type { ChunkRow, StorageRecord } from './chunk-rows.ts'
 export type { SessionSurface, SurfaceFoldReplacement, SurfaceFoldResult } from './surface.ts'