chore(config): set default model to Opus 4.8 (1M context)

Pin the default model to claude-opus-4-8[1m] in settings.json (the tracked
config deployed to ~/.claude). Previously no model key was committed, so the
default resolved to the tier default; a working-tree pin to claude-fable-5[1m]
was never committed. Takes effect at next session start.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XpphkdTosUzokBDNG7PToS

settings.json

@@ -225,6 +225,7 @@
     "disableBypassPermissionsMode": "disable",
     "additionalDirectories": []
   },
+  "model": "claude-opus-4-8[1m]",
   "hooks": {
     "SessionStart": [
       {