chore(hygiene): guard the root session-handoff entry point

The working handoff now lives at `/_NEXT_SESSION.md` (root) instead of
`docs/NEXT_SESSION.md`, so that a fresh session's first-pass scan of the project
root finds it without being told where to look. It is kitchen, not product: add
it to .gitignore and teach the hygiene gate to flag it, the same way the gate
already flags root audits, planning notes and the old docs/ handoff. Self-test
covers the new pattern (9 flagged / 7 clean).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: JuliaEdom

.gitignore

@@ -116,6 +116,7 @@ reviews/*.err
 /plan_*.md
 /_ref_*
 /[0-9][0-9]_[0-9][0-9]_[0-9][0-9].md
+/_NEXT_SESSION.md
 /docs/NEXT_SESSION.md
 /docs/SESSION_HANDOFF.md
 /docs/data_flow.*

scripts/check_repo_hygiene.py

@@ -40,6 +40,8 @@ def classify(path: str) -> str | None:
             return "internal root-level planning note"
         if path.startswith("_ref_"):
             return "internal root-level reference scratch"
+        if path.startswith("_NEXT_SESSION"):
+            return "internal session handoff (root entry point)"
         if _DATE_NOTE.match(path):
             return "internal root-level dated session note"
     if path == "docs/NEXT_SESSION.md":
@@ -81,6 +83,7 @@ def self_test() -> int:
         "plan_for_pres.md",
         "_ref_presentation3.html",
         "27_05_26.md",
+        "_NEXT_SESSION.md",
         "docs/NEXT_SESSION.md",
         ".claude/settings.json",
         "scratchpad/tmp.py",