fix(provider): make config.yaml model.provider the single source of truth (#31222)

Policy: if it ain't a secret it goes in config.yaml. HERMES_INFERENCE_PROVIDER
was leaking behavioral config into the .env surface, including from the gateway,
which bypassed config.yaml entirely.

Behavior:
- gateway/run.py: drop HERMES_INFERENCE_PROVIDER read in _resolve_runtime_agent_kwargs.
  Gateway now flows through resolve_runtime_provider() with no `requested` override,
  which reads model.provider from config.yaml first.

Docs/UX (strip env var from user-facing surface):
- --provider help text no longer mentions the env var
- cli-config.yaml.example same
- reference/environment-variables.md: remove HERMES_INFERENCE_PROVIDER row and
  the cross-reference from HERMES_INFERENCE_MODEL
- reference/cli-commands.md: blank the env-var column for --provider
- guides/xai-grok-oauth.md, guides/minimax-oauth.md: replace
  HERMES_INFERENCE_PROVIDER=x hermes invocations with config.yaml / --provider
- developer-guide/adding-providers.md, model-provider-plugin.md: reframe

Internal mechanism (kept as-is):
- hermes_cli/main.py writes HERMES_INFERENCE_PROVIDER into the TUI subprocess env
- tui_gateway/server.py reads it on TUI startup
- resolve_requested_provider() / oneshot.py / cli.py still fall through to the
  env var as a last-resort behind config.yaml, which is what makes the TUI
  parent->child handoff work
This stays. We just stop documenting it as a user knob.

Tests: tests/gateway/test_auth_fallback.py — simplify mock to fail on first
call, succeed on second; drop monkeypatch.setenv lines that no longer matter.

Supersedes #31064 (closed with credit to @novax635 who surfaced the underlying
issue but proposed aligning gateway *to* the env var rather than removing it).

gateway/run.py

@@ -962,6 +962,12 @@ _AGENT_PENDING_SENTINEL = object()
 def _resolve_runtime_agent_kwargs() -> dict:
     """Resolve provider credentials for gateway-created AIAgent instances.
 
+    Provider is read from ``config.yaml`` ``model.provider`` (the single
+    source of truth). ``resolve_runtime_provider()`` falls through to env
+    var lookups internally for legacy compatibility, but the gateway does
+    not consult environment variables for behavioral config — config.yaml
+    is authoritative.
+
     If the primary provider fails with an authentication error, attempt to
     resolve credentials using the fallback provider chain from config.yaml
     before giving up.
@@ -973,9 +979,7 @@ def _resolve_runtime_agent_kwargs() -> dict:
     from hermes_cli.auth import AuthError
 
     try:
-        runtime = resolve_runtime_provider(
-            requested=os.getenv("HERMES_INFERENCE_PROVIDER"),
-        )
+        runtime = resolve_runtime_provider()
     except AuthError as auth_exc:
         # Primary provider auth failed (expired token, revoked key, etc.).
         # Try the fallback provider chain before raising.