Merge branch 'master' into worktree-llm-tool-order

docs/core-data-structures/session.md

@@ -200,6 +200,14 @@ export interface SurfaceNode {
 
 Everything else (`turn/*`, `step/*`) is structural and does not project into a message. Token usage is observed on `assistant/message.usage` (the step that produced it); an operational error's step number is on `turn/end.reason` for `kind: 'error'`.
 
+## Live-session fork API
+
+`ctx.sessions.create(id, { seed, meta })` is the low-level replay/fork primitive. For ordinary live-session forks, `SessionStore` exposes one policy API:
+
+- `fork(source, boundary?, childSessionId?)` accepts a live `Session` object or live `SessionId`, selects source events through the inclusive `boundary` seq (default: current last event), requires the boundary event to be `turn/end`, then creates a live child session with deep-cloned seed events plus child metadata (`parentSession`, `seedLength`, and inherited `cwd`).
+
+An explicit `boundary` lets callers fork from a previous completed turn even if the source has newer events or an open current turn. The API rejects non-`turn/end` boundaries instead of clipping silently. Broader turn-enclosure sanity stays in the existing `dsh-invariants` plugin and persistence repair path rather than being duplicated in `fork()`. `dsh-subagent-fork` keeps its completed-prefix clipping because tool-time delegation usually starts while the parent turn is open; ordinary session branching should make the requested boundary explicit.
+
 ## What started a turn: `TurnTriggerMap`
 
 ```ts type-equiv