D756 — A routed ?kbs= serves the mounted subset and reports the unknown names

Decision. On /mcp/routed, the effective set is ?kbs= intersected with the mounted KBs. Names that are not mounted are reported, not dropped silently: kb_status.connection.unknown, an extended hint, the error of a call naming one (pointing at cartographer sync), and one server-side log line per distinct (set, unknown) pair. When no name is mounted the request stays 400, listing every unknown name sorted and the mounted KBs. Amends D288.

Why. A client URL written before a kb rename or an unmount made every request fail, so the agent lost the mounted KBs too and had no hint. The mount set changes only across a restart, so a name becomes unknown exactly then. D288 rejected silent dropping because a typo would hide a KB without a trace; reporting through the session channel D712 added keeps that guarantee.

Alternatives rejected. Keep the 400 (loses every KB for one stale name). Fall back to all mounted KBs when nothing matches (widens a binding that only narrows). Empty set (a server with no tools). A warning in initialize instructions (the server sends none, and an open session has consumed them). Aliasing old to new names (needs a renamed_from record and changes kb rename, D177): a separate follow-up.

Consequences. A stale URL now connects instead of failing with 400. Unknown names are request-scoped (request context), never stored on the cached per-set Server, so URLs differing only in stale names share one view. An empty effective set must stay an error (comment in routedFor).