[Bug] Advisor tool returns generic "unavailable" error for unsupported model pairing without diagnostic

Bug Description
Title: advisorModel: "fable" → advisor tool "unavailable" (no usable error / undocumented setting)

Environment:

• Claude Code v2.1.170 (CLI)
• Session executor / main model: Opus 4.8 (claude-opus-4-8[1m], 1M context)
• ~/.claude/settings.json: "advisorModel": "fable", "effortLevel": "xhigh"

Repro:

1. Set "advisorModel": "fable" in settings.json.
2. Run a session on the Opus 4.8 model.
3. Invoke the built-in advisor() tool.

Actual:

• advisor() returns: "The advisor tool is unavailable. Do not try to use it again." …

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