docs(sightsound): correct unknown-model-tag contract to live-probed behavior

Prior docs claimed unknown tags silently serve llama3.2:1b. Live probes on the
live endpoint disprove it: /v1/chat/completions answers HTTP 200 with a flat
{"error":"Failed to load <tag> model!"} body and no choices; /v1/completions
answers a nested OpenAI-shaped server_error (code 500) under HTTP 200. Valid
tags route per-request to the named model (response echoes the requested tag);
no restart needed to switch between registry tags. Corrected in the runbook,
the AGENTS.md runtime section, the serve-script header comment, and the
historical FastFlowLM incident note.

scripts/serve-frank-default-sightsound.sh

@@ -9,8 +9,9 @@
 # for the memory budget and the model-selection tradeoff.
 #
 # NOTE: FLM requires unlimited memlock. The unit sets LimitMEMLOCK=infinity.
-# NOTE: an unknown model tag does NOT 404 — FLM logs an error and silently
-# serves llama3.2:1b instead. Client model IDs must be the exact FLM registry
+# NOTE: an unknown model tag does NOT 404 — /v1/chat/completions answers HTTP 200 with a
+# flat {"error":"Failed to load <tag> model!"} body and no choices; /v1/completions returns a
+# nested OpenAI-shaped server_error (code 500) also under HTTP 200. Client model IDs must be the exact FLM registry
 # tag (e.g. gemma4-it:e4b), never a service alias. Custom tags only resolve
 # when FLM_CONFIG_PATH points at the user registry that carries them and their
 # kernels are symlinked into /home/dank/.config/flm/xclbins/<PackName>.