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