Add a safe harness for nested session tests; ignore the cosmic-session fork

Running a second cosmic-session on the development machine turns out to be
genuinely dangerous, and twice it logged the developer out mid-session and
destroyed open work. Two distinct causes, both encoded here as guards:

  - Name-based process selection cannot distinguish the fork, the system
    install, or a stand-in binary; they all answer to `cosmic-session`. The
    second logout came from `pgrep -x cosmic-session | head -1` inside the
    test written to demonstrate that name matching is unsafe, because `head
    -1` favours the oldest match, which is always the live desktop. The
    harness therefore never selects a process by name: it spawns under
    setsid and signals `-$PGID`, with the group ID taken from `$!`.

  - A nested cosmic-session takes the well-known D-Bus name
    com.system76.CosmicSession away from the running session on a shared bus
    ("Connection `:1.3` lost name ..." in the journal), destabilising the
    outer desktop before anything is killed. The harness always runs under
    dbus-run-session. Nesting cosmic-comp alone does not need this.

It also refuses to start without WAYLAND_DISPLAY, since the winit backend
would otherwise fall back to DRM and seize the real display, and it reaps
IPC socket directories whose owning PID is gone.

Verified: shellcheck-clean syntax; the no-WAYLAND_DISPLAY guard fires; an
audit confirms every `kill` targets the script's own process group and no
code path matches a process by name. NOT verified: the harness has never
been run against the real binaries. Session-level runtime testing is now
deferred to a VM or to logging into hyprcosmic.desktop directly, rather than
nesting inside the developer's live desktop.

cosmic-session joins cosmic-comp in .gitignore; both are forks that become
submodules under the topology in the design spec.

.gitignore

@@ -6,5 +6,6 @@ target/
 **/*.rs.bk
 vendor/
 
-# Fork; becomes a submodule per spec repo topology
+# Forks; become submodules per spec repo topology
 cosmic-comp/
+cosmic-session/