fix(issue-50): an injected record holds its session whatever its FIFO place; the ambient pane claim locks the caller's server (reviews 5a09a13f F-A, e8e471ff R1)

open_jobs reports a live pending record nobody signals on as `injected`:
its producer registered and injected it without a waiter (create
--onboard, a job sent by hand), so it holds the session like a running or
admitted job, and a waiter of the same second with a lower job id, or one
registered a moment earlier and still in its preflight, no longer passes
it in the FIFO. The waiter signals on its own record before its first
scan, so its record is never mistaken for one.

lib_py.pane_lock resolves the shim's ambient target (no -L,
HERDR_SESSION_NAME empty or `default`) to the server of the pane the
caller runs in - HERDR_SESSION, else HERDR_SOCKET_PATH's layout, as
lib.sh _herdr_caller_server reads them - so an ambient and a named call
into one server claim under one lock; outside a pane it stays the
default server's.

Tests: u2h (the tie, the later-registered onboarding, the signalled
control), u1/u4 labels, c10 forces the same-second registration, p1b,
s2 (real herdr: held lock stops both call forms; mixed concurrent calls
take distinct panes). Two FIFO fixtures now carry a waiter's signal.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.agents/skills/lib.sh

@@ -1501,7 +1501,9 @@ _herdr_scope_is_pane_inherited() {
 # in the shim heredoc, which stays byte-identical (test_issue23_24 S11).
 #
 # _mam_pane_claim_target <herdr argv...>: prints the server a new-session call
-# targets ("" for the default one); rc 1 for any other call.
+# names ("" for the ambient target, which lib_py.pane_lock resolves to the
+# caller pane's server - the one herdr answers on - or the default server);
+# rc 1 for any other call.
 _mam_pane_claim_target() {
   local sess="${HERDR_SESSION_NAME:-}"
   while [ "${1:-}" = "-L" ]; do sess="${2:-}"; shift 2 || return 1; done