fix review findings: CI leaf-gate wiring, heritage return surface, AGENTS.md self-containedness

- run-gates.ts docSyncLeafGates() gains verify-export-jsdoc — CI lanes
  and the pre-push hook execute this leaf list, not the doc-sync npm
  script, so the gate was previously unenforced there (proven by
  SessionForkErrorCode landing undocumented via a master merge while
  checks stayed green; now documented). Same wiring gap fixed for
  master's verify-config-catalog, which was also missing from the list.
- The heritage exemption now recovers the base's return surface: a void
  base return carried no @returns duty, so an override returning a
  concrete result documents it itself (annotated overrides run the
  standard check; unannotated ones are classified by the checker so
  faithful void overrides need no boilerplate annotation). Three new
  negative-path tests pin it; RFC and module doc updated.
- AGENTS.md states each principle inline instead of citing RFCs (eight
  citations removed; high-level doc links kept) and the editing section
  now carries the self-containedness rule.
- Generated catalogs/graphs regenerated for the shifted line pointers.

packages/core/session/src/index.ts

@@ -373,6 +373,14 @@ export class Session {
 /** A fork source: either the live session object or its live store id. */
 export type SessionForkSource = Session | SessionId
 
+/**
+ * Rejection codes for session forking: the fork source id is unknown to the
+ * live store (`SESSION_NOT_FOUND`) or names a session object that is not the
+ * store's live instance (`SESSION_NOT_LIVE`); the requested child id is
+ * already taken (`SESSION_ALREADY_EXISTS`); the boundary is not a contiguous
+ * existing seq (`INVALID_BOUNDARY`); or the boundary event is not a
+ * `turn/end` — a fork must cut on a closed turn (`OPEN_TURN`).
+ */
 export type SessionForkErrorCode =
   | 'SESSION_NOT_FOUND'
   | 'SESSION_NOT_LIVE'