made extension tmux-only, replaced cmux.ts with tmux.ts (~1368 -> 354 lines)

Removed the cmux, zellij, and wezterm backends, the MuxBackend type, the
PI_SUBAGENT_MUX override, and dead exports (renameCurrentTab, renameWorkspace,
sendEscape, isFishShell, exitStatusVar). The ten-function surface API used by
index.ts is unchanged. Integration harness and tests are tmux-only; README and
package.json now state tmux-only with a fork note pointing multi-multiplexer
users at the upstream project.

test/integration/subagent-lifecycle.test.ts

@@ -2,14 +2,13 @@
  * Integration tests for the full subagent lifecycle.
  *
  * These tests spawn REAL pi sessions with REAL LLM calls (haiku by default).
- * Each test creates a mux surface, runs pi with a task that uses the subagent
+ * Each test creates a tmux pane, runs pi with a task that uses the subagent
  * tool, and verifies the outcome via marker files and screen output.
  *
  * Costs: ~$0.01-0.05 per test run (haiku).
  * Duration: ~30-90s per test.
  *
- * Run inside a supported multiplexer:
- *   cmux bash -c 'npm run test:integration'
+ * Run inside tmux:
  *   tmux new 'npm run test:integration'
  *
  * Configuration:
@@ -21,8 +20,6 @@ import assert from "node:assert/strict";
 import { existsSync, readFileSync, unlinkSync } from "node:fs";
 import {
   getAvailableBackends,
-  setBackend,
-  restoreBackend,
   createTestEnv,
   cleanupTestEnv,
   createTrackedSurface,
@@ -40,23 +37,20 @@ import {
 const backends = getAvailableBackends();
 
 if (backends.length === 0) {
-  console.log("⚠️  No mux backend available — skipping subagent lifecycle integration tests");
-  console.log("   Run inside cmux or tmux to enable these tests.");
+  console.log("⚠️  tmux is not available — skipping subagent lifecycle integration tests");
+  console.log("   Run inside tmux to enable these tests.");
 }
 
 for (const backend of backends) {
   describe(`subagent-lifecycle [${backend}]`, { timeout: PI_TIMEOUT * 3 }, () => {
-    let prevMux: string | undefined;
     let env: TestEnv;
 
     before(() => {
-      prevMux = setBackend(backend);
-      env = createTestEnv(backend);
+      env = createTestEnv();
     });
 
     after(() => {
       cleanupTestEnv(env);
-      restoreBackend(prevMux);
     });
 
     // ── Basic spawn + completion ──