Reported during the TUI v2 blitz test: switching from openrouter to
anthropic via `/model <name> --provider anthropic` appeared to succeed,
but the next turn kept hitting openrouter — the provider the user was
deliberately moving away from.
Two gaps caused this:
1. `Agent.switch_model` reset `_fallback_activated` / `_fallback_index`
but left `_fallback_chain` intact. The chain was seeded from
`fallback_providers:` at agent init for the *original* primary, so
when the new primary returned 401 (invalid/expired Anthropic key),
`_try_activate_fallback()` picked the old provider back up without
informing the user. Prune entries matching either the old primary
(user is moving away) or the new primary (redundant) whenever the
primary provider actually changes.
2. `_apply_model_switch` persisted `HERMES_MODEL` but never updated
`HERMES_INFERENCE_PROVIDER`. Any ambient re-resolution of the runtime
(credential pool refresh, compressor rebuild, aux clients) falls
through to that env var in `resolve_requested_provider`, so it kept
reporting the original provider even after an in-memory switch.
Adds three regression tests: fallback-chain prune on primary change,
no-op on same-provider model swap, and env-var sync on explicit switch.
