feat(tmux): add killTmuxSessionIfExists utility for explicit session teardown

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.

src/shared/tmux/tmux-utils/session-spawn.ts

@@ -6,7 +6,7 @@ import { isInsideTmux } from "./environment"
 import { isServerRunning } from "./server-health"
 import { shellEscapeForDoubleQuotedCommand } from "../../shell-env"
 
-const ISOLATED_SESSION_NAME = "omo-agents"
+export const ISOLATED_SESSION_NAME = "omo-agents"
 
 async function getWindowDimensions(
 	tmux: string,