Add model_fallback to the plugin index

[Olszalsik]

Adds Model Fallback System (model_fallback).

Multi-cycle model failover for Agent Zero. When a provider rate-limits, times out or
5xxs, the plugin rotates through fallback model candidates using shared cooldowns,
permanent-failure detection, latency-learned warm/cold timeouts, background recovery
probes and an extended-retry mode for hour-long quota resets. It covers the chat,
utility and main-loop turn paths, and modifies no core agent-zero file.

How this differs from the related entries

• api_rotation_by_kame rotates API keys within a single provider. This plugin
  rotates across model candidates, classifies permanent vs transient failures, and
  learns per-label timeouts from real call outcomes.
• apu_governor is a global tool-execution throttle that paces calls to avoid
  429s. This plugin is failure-driven: it only rotates when a provider actually
  errors or rate-limits.
• a0_vision_fallback routes image analysis to a vision model; unrelated scope.

Verification

• scripts/validate_plugin_submission.py passes locally against this exact
  BASE_SHA/HEAD_SHA range.
• Plugin repository: https://github.com/Olszalsik/a0-model-fallback
  (public, default branch main, root plugin.yaml with name: model_fallback).
• Plugin test suite: 336 passing.
• No secrets in the working tree or in any of the repository's 30 commits;
  config.json (per-install runtime config) is git-ignored and was never committed.

[POWERFULMOVES]

Verified: index-only addition, validate CI green. Upstream pattern matches established provider-fallback plugins; fills a real gap (model failover on rate-limit/5xx). Safe index entry.