fix(ai): move the Codex slug ladder onto the gpt-5.6 line

The ChatGPT Codex backend now rejects every slug from gpt-5.5 down, so a
deploy without a CODEX_MODEL_FALLBACK_CHAIN override walked the whole
default ladder into "all configured Codex slugs were rejected". The
default now starts at gpt-5.6-terra, with its luna and sol siblings and
gpt-5.5 as the last rung.

The gpt-6 line stays out of the default: the reasoning, sampling and
vision gates do not recognise that family yet, and a test now holds every
default slug to a family the reasoning gate knows.

src/__tests__/baselines/source-file-lines.json

@@ -1,7 +1,7 @@
 {
   "$comment": "Production files over 800 lines and their length. Maintained by src/__tests__/source-size-ratchet.test.ts.",
   "count": 89,
-  "total": 99598,
+  "total": 99593,
   "entries": {
     "app/medications/page-client.tsx": 839,
     "components/charts/mood-chart.tsx": 1166,
@@ -29,7 +29,7 @@
     "lib/ai/coach/stream-events.ts": 817,
     "lib/ai/coach/turn/model.ts": 820,
     "lib/ai/coach/types.ts": 1011,
-    "lib/ai/codex-client.ts": 1094,
+    "lib/ai/codex-client.ts": 1089,
     "lib/ai/local-client.ts": 880,
     "lib/ai/prompts/insight-generator.ts": 1110,
     "lib/ai/provider.ts": 1632,