Adds killTmuxSessionIfExists(sessionName), a best-effort no-op when the
named session is absent. Drains both stdio streams so it does not leak
pipe buffers the way closeTmuxPane historically did.
Also exports ISOLATED_SESSION_NAME ("omo-agents") from session-spawn so
callers can tear down the shared isolated session without hard-coding
the name in multiple places.
YeonGyu-Kim