test(conftest): strip HERMES_WEBUI_SKIP_ONBOARDING env globally; rfcs: note discussion-first for contributor RFCs

Two follow-ups from Opus pre-release review of stage-336:

1. tests/conftest.py — autouse session fixture that removes
   HERMES_WEBUI_SKIP_ONBOARDING from os.environ for the whole pytest run, and
   restores it after. Hosting providers and isolated harnesses set this var
   to short-circuit the onboarding wizard, but it leaked into pytest and
   caused tests that exercise apply_onboarding_setup() to fail with cryptic
   FileNotFoundError. Tests that specifically validate the short-circuit
   behavior can opt back in with monkeypatch.setenv. Surgical per-test
   delenv calls remain as defense-in-depth but are now redundant.

2. docs/rfcs/README.md — one-line note that first-time contributor RFCs
   should be discussed in an issue before opening a PR. Gates drive-by
   design-doc PRs without us having to decline them on contribution.

Verified: 96 onboarding-related tests pass with HERMES_WEBUI_SKIP_ONBOARDING=1
exported in the test runner env (would have failed before this fixture).

docs/rfcs/README.md

@@ -28,6 +28,7 @@ cutting infrastructure.
 - A reviewer asks for one during code review.
 
 When in doubt, just ship the code — small features don't need RFCs.
+First-time contributor RFCs should be discussed in an issue before opening a PR.
 
 ## Current RFCs
 

tests/conftest.py

@@ -152,6 +152,24 @@ def pytest_configure(config):
     config.addinivalue_line("markers", "requires_agent: skip when hermes-agent dir is not found")
     config.addinivalue_line("markers", "requires_agent_modules: skip when hermes-agent Python modules are not importable")
 
+
+# ── Environment isolation for tests ────────────────────────────────────────
+# HERMES_WEBUI_SKIP_ONBOARDING is set by hosting providers (e.g. Agent37) and
+# by some isolated test harnesses to short-circuit the onboarding wizard.
+# When it leaks into the pytest environment, tests that exercise the wizard
+# code paths (apply_onboarding_setup, etc.) fail because the function returns
+# early without writing config files.
+#
+# This autouse fixture removes the variable for the test session. Tests that
+# specifically need to validate the SKIP_ONBOARDING short-circuit can opt back
+# in with `monkeypatch.setenv("HERMES_WEBUI_SKIP_ONBOARDING", "1")`.
+@pytest.fixture(autouse=True, scope="session")
+def _strip_skip_onboarding_env():
+    prior = os.environ.pop("HERMES_WEBUI_SKIP_ONBOARDING", None)
+    yield
+    if prior is not None:
+        os.environ["HERMES_WEBUI_SKIP_ONBOARDING"] = prior
+
 def pytest_collection_modifyitems(config, items):
     """Auto-skip agent-dependent tests when hermes-agent is not available.