fix: report a clear cli error when no herdr server is running (#1963)

* fix: report a clear cli error when no herdr server is running

socket cli commands surfaced a raw io::Error debug string
(`Error: Os { code: 2, ... }`) when nothing was listening on the
api socket, which read like a bad --cwd path. map dead-socket
connect failures to a `server_not_running` json error carrying the
resolved socket path, printed once at the edge that surfaces the
error; recovering callers (plugin offline registry fallback, agent
start polling) recognize the marker and keep their existing
behavior.

refs #1941

* fix: map dead-socket errors on the unchecked cli request path

refs #1941

* fix: make server-not-running guidance session-aware

refs #1941

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