chore(release): 6.0.0 — Voyage never starts Claude Code

Valgt 6.0.0 (major) because the change is incompatible for a 5.11.1 user:
/trekexecute multi-session no longer finishes in one call, /trekplan
--decompose no longer writes launch.sh, and the per-session turn/budget
environment variables no longer apply. Version synced in plugin.json,
package.json, package-lock.json (both fields), the README badge and the
CHANGELOG heading.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "voyage",
-  "version": "5.11.1",
+  "version": "6.0.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "voyage",
-      "version": "5.11.1",
+      "version": "6.0.0",
       "license": "MIT",
       "engines": {
         "node": ">=18"