feat(advisor): document fallback when advisorModel is unavailable

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Problem

When advisorModel: "fable" is configured in settings but Fable is unavailable (model outage, rate limit, etc.), the wizard provides no guidance on what to use as a fallback for cross-model review.

Current behavior

User has to figure out on their own to switch to Codex (GPT 5.5 xhigh) or another reviewer. No error handling, no docs, no wizard guidance.

Expected behavior

The wizard should:

1. Document the fallback procedure in the wizard doc (e.g., "if advisor is unavailable, use codex exec with GPT 5.5 xhigh for cross-model review")
2. Optionally: detect advisor unavailability at session start and surface the fallback recommendation
3. The /update skill could check advisor health and suggest alternatives

Context

Discovered June 19, 2026 when Fable advisor was unavailable during an anticheat session. User had to manually recall the Codex fallback procedure. This should be documented so it's not a tribal-knowledge gap.

Acceptance criteria

• Wizard doc has a "Fallback Reviewers" section under cross-model review
• Fallback hierarchy documented: Fable > Codex GPT 5.5 xhigh > Agent(model="opus") spawn
• Optional: SessionStart hook detects advisor unavailability

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Triage 2026-06-22: Advisor fallback is already documented in skills/sdlc/SKILL.md lines 109+139 (PR #412). Verify the docs match current behavior, close if redundant. Next action: verify advisor fallback docs against current CC behavior. Status: low priority, may be already-done.