fix: detect Mercury stage per-view via session name

The Mercury app (3.2.x) is a hybrid server whose kernel pool serves both
the client-facing app view and JupyterLab, so the server-level
MERCURY_CONFIG_DIR signal cannot tell who is looking and leaks backstage
content in the app. Switch the primary stage signal to the shadow-copy
session name (__mercury__ in JPY_SESSION_NAME), keeping the env var only
as a --working-dir fallback.

- Update CX Discovery staging.py to the per-view detection model
- Ignore Mercury runtime artifacts (.mercury_sessions/, *__mercury__*)
- Document the leak in remaining masters (TEI, CTM, AI Diagnostic,
  template) and flag the staging.py retrofit as urgent

.gitignore

@@ -9,6 +9,11 @@
 .env
 .DS_Store
 
+# Mercury app runtime artifacts — per-session shadow notebook copies
+# (in an engagement copy these hold live session state: never commit)
+.mercury_sessions/
+*__mercury__*
+
 # Generated exports — never committed (masters and assessments)
 studies/*/exports/*
 !studies/*/exports/.gitkeep