fix(gateway): reload fallback chain when config changes

[Que0x]

What & why

The gateway loaded the fallback provider chain once at __init__
(self._fallback_model) and never refreshed it. The agent-cache signature
also omitted the chain, so an operator who edited fallback_providers /
fallback_model in config.yaml kept getting routed on the stale chain —
even on a brand-new session — until a full gateway restart. This contradicts
the documented "changes take effect on the next agent session or gateway
restart" contract (web-dashboard.md), and means a provider auth/rate-limit
fix doesn't actually take hold.

How

All changes in gateway/run.py:

• _load_fallback_model(cfg=None) — accepts a pre-loaded config so the
  chain can be re-resolved from the current turn's config (no extra disk read).
  Backward compatible; the __init__ call is unchanged.
• _extract_cache_busting_config — folds the effective merged chain
  (fallback.chain) into the agent-cache signature, so a change rebuilds the
  cached agent and an unchanged chain causes no cache churn.
• _run_agent / _run_background_task — re-resolve
  self._fallback_model from the freshly-loaded user_config before building
  the agent.

Legacy fallback_model + fallback_providers merge semantics are preserved
(get_fallback_chain). No behavior change for users without a fallback config
(empty chain → None).

How to test

pytest tests/gateway/test_agent_cache.py \
       tests/gateway/test_auth_fallback.py \
       tests/gateway/test_session_model_override_routing.py -q

Tests & results

Suite	Result
test_agent_cache.py + test_auth_fallback.py + test_session_model_override_routing.py	76 passed
└ new TestFallbackChainCacheBusting (regression)	7 passed
test_background_command.py (covers edited _run_background_task)	22 passed

New regression tests assert: signature busts when the chain changes,
stays stable when unchanged (no churn), legacy fallback_model merge is
preserved, and _load_fallback_model(cfg) resolves from a passed config.

[teknium1]

Thanks for the careful fallback-chain/cache analysis. This is an automated hermes-sweeper review; current main now provides the requested behavior through a later, stronger implementation.

• be1346cf2a0edb6e0f64bee3f5ce10d470e4fb33 reloads the configured fallback chain for fresh gateway agents, background tasks, and cached-agent reuse.
• gateway/run.py:5016 now rereads the chain from config.yaml; gateway/run.py:18258 applies the refreshed chain to an already-cached agent outside the cache lock, preserving cache reuse rather than rebuilding solely for this configuration change.
• 0a01b2087d0c0bb11bb7ed650b8de3e6b5205856 hardens that path by retaining the last known-good chain during transient parse/read failures and clearing stale unavailable-provider memoization after a real chain edit.
• tests/gateway/test_fallback_chain_reload.py:18 covers live reload, cached-agent application, background/new-agent construction paths, cooldown handling, and torn-write behavior.

Closing as implemented on main.