fix: Windows guard, dedup recovery, profile-safe paths, clear SO_RCVTIMEO

Follow-up fixes for salvaged PR #67686 (issue #67639):

1. Windows regression: bare 'import fcntl' at module level in entry.py and
   slash_worker.py would crash on Windows (fcntl is POSIX-only). entry.py
   explicitly aims to 'import cleanly on Windows'. Extracted all fcntl/socket
   logic to tui_gateway/_stdin_recovery.py with try/except import guards.

2. fd leak: socket.fromfd() dups fd 0; s.detach() returned the fd number
   without closing it, leaking one fd per recovery call. Changed to s.close()
   (safe — fromfd duped the fd, closing won't close stdin).

3. Code duplication: ~80 lines of recovery loop + diagnostic were copy-pasted
   between entry.py and slash_worker.py. Extracted to shared
   tui_gateway/_stdin_recovery.py (handle_spurious_eof + diagnose_stdin_state).

4. Profile-unsafe path: scanner used Path.home() / '.hermes' / 'plugins' but
   canonical plugin discovery uses get_hermes_home() / 'plugins'. With
   HERMES_HOME pointing elsewhere, scanner missed the actual plugin dir.
   Also gated project plugins behind HERMES_ENABLE_PROJECT_PLUGINS to match
   hermes_cli/plugins.py.

5. SO_RCVTIMEO not cleared: recovery only called os.set_blocking(0, True)
   but a child that set SO_RCVTIMEO would cause the next readline to time
   out and loop. Now clears SO_RCVTIMEO alongside O_NONBLOCK.

scripts/check_subprocess_stdin.py

@@ -38,13 +38,15 @@ TUI_CONTEXT_DIRS = [
 ]
 
 # User plugin roots — scanned at runtime if they exist.  Plugins load from
-# ~/.hermes/plugins/ and ./.hermes/plugins/ (hermes_cli/plugins.py:10-12),
-# but the guard only checked the bundled plugins/ dir, missing user-installed
-# code that spawns subprocesses (gap reported in #67639).
-USER_PLUGIN_DIRS = [
-    Path.home() / ".hermes" / "plugins",
-    Path.cwd() / ".hermes" / "plugins",
-]
+# ``get_hermes_home() / "plugins"`` (user) and ``./.hermes/plugins/`` (project,
+# gated behind ``HERMES_ENABLE_PROJECT_PLUGINS``) — see
+# ``hermes_cli/plugins.py:10-12``.  The guard only checked the bundled
+# ``plugins/`` dir, missing user-installed code that spawns subprocesses
+# (gap reported in #67639).
+#
+# Import is deferred to ``main()`` (after ``os.chdir(repo_root)``) because
+# this script runs as a standalone subprocess — ``hermes_constants`` isn't
+# on ``sys.path`` until the repo root is added.
 
 # subprocess and os APIs that inherit stdin by default when called without
 # an explicit stdin= argument.  The original regex only covered run/Popen
@@ -155,6 +157,11 @@ def main() -> int:
     repo_root = Path(__file__).resolve().parent.parent
     os.chdir(repo_root)
 
+    # Add repo root to sys.path so we can import hermes_constants (this script
+    # runs as a standalone subprocess, not as a module).
+    sys.path.insert(0, str(repo_root))
+    from hermes_constants import get_hermes_home
+
     all_violations = []
 
     for tui_dir in TUI_CONTEXT_DIRS:
@@ -178,11 +185,15 @@ def main() -> int:
             violations = find_subprocess_calls(content, rel)
             all_violations.extend(violations)
 
-    # Scan user plugin directories (Gap 1: guard missed ~/.hermes/plugins/
-    # and ./.hermes/plugins/, where user-installed plugins like ori/hooks.py
-    # can spawn subprocesses with inherited stdin — #67639).
+    # Scan user plugin directories (Gap 1: guard missed user-installed
+    # plugins in get_hermes_home()/plugins/ and project plugins in
+    # ./.hermes/plugins/, where code like ori/hooks.py can spawn
+    # subprocesses with inherited stdin — #67639).
+    plugin_roots: list[Path] = [get_hermes_home() / "plugins"]
+    if os.environ.get("HERMES_ENABLE_PROJECT_PLUGINS"):
+        plugin_roots.append(Path.cwd() / ".hermes" / "plugins")
     seen_roots: set[Path] = set()
-    for plugin_root in USER_PLUGIN_DIRS:
+    for plugin_root in plugin_roots:
         resolved = plugin_root.resolve()
         if resolved in seen_roots or not resolved.is_dir():
             continue