fix(apiproxy): harden cold session metadata probing

docs/subsystems/session.md

@@ -586,7 +586,7 @@ An explicitly supplied empty seed writes `session/end-seed` at seq 0, which dist
 
 It exists because seed history and live work are otherwise byte-identical, which defeats any plugin owning a standalone open/close bracket: an unmatched `compaction/start` reads the same whether the writer crashed mid-compaction or is compacting right now. An opening marker before `session/end-seed` came from the constructor seed and belongs to an ended lifecycle, whatever ended it (a crash, a succeeding process, or a fork out of a still-running parent), so its owner may treat it as dead. That covers only brackets *this* session inherited: a concurrently live session holding an open bracket over the same history has its own boundary elsewhere, so tolerating concurrent writers needs a liveness signal beyond the log. Core writes the boundary and reads nothing from it — a bracket's vocabulary stays with its owning plugin, which is why crash repair closes turn/step/tool boundaries and never `compaction/*`.
 
-Activity ordering excludes the boundary through `lastActivityTime(events)`: picking a session up is not work, and lazy resume means browsing writes one, so a resume picker or session list ordering by log tail would float every opened session to the top.
+Consumers that order Sessions by human activity exclude this boundary: picking a Session up is not work, so ordering by the log tail would float every opened Session to the top.
 
 ## Plugin-contributed log-only events