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-kill.ts

@@ -0,0 +1,51 @@
+async function readStream(stream: ReadableStream<Uint8Array> | null | undefined): Promise<string> {
+	return stream ? new Response(stream).text() : ""
+}
+
+export async function killTmuxSessionIfExists(sessionName: string): Promise<boolean> {
+	const [{ log }, { isInsideTmux }, { getTmuxPath }, { spawn }] = await Promise.all([
+		import("../../logger"),
+		import("./environment"),
+		import("../../../tools/interactive-bash/tmux-path-resolver"),
+		import("./spawn-process"),
+	])
+
+	if (!isInsideTmux()) {
+		log("[killTmuxSessionIfExists] SKIP: not inside tmux", { sessionName })
+		return false
+	}
+
+	const tmux = await getTmuxPath()
+	if (!tmux) {
+		log("[killTmuxSessionIfExists] SKIP: tmux not found", { sessionName })
+		return false
+	}
+
+	const hasSessionProcess = spawn([tmux, "has-session", "-t", sessionName], {
+		stdout: "ignore",
+		stderr: "ignore",
+	})
+
+	if ((await hasSessionProcess.exited) !== 0) {
+		log("[killTmuxSessionIfExists] SKIP: session not found", { sessionName })
+		return false
+	}
+
+	const killSessionProcess = spawn([tmux, "kill-session", "-t", sessionName], {
+		stdout: "pipe",
+		stderr: "pipe",
+	})
+	const [, stderr, exitCode] = await Promise.all([
+		readStream(killSessionProcess.stdout),
+		readStream(killSessionProcess.stderr),
+		killSessionProcess.exited,
+	])
+
+	if (exitCode !== 0) {
+		log("[killTmuxSessionIfExists] FAILED", { sessionName, exitCode, stderr: stderr.trim() })
+		return false
+	}
+
+	log("[killTmuxSessionIfExists] SUCCESS", { sessionName })
+	return true
+}