chore(skills): rename the worktree profile to fleet

The profile's distinguishing trait is a branch and PR per issue run by
several workers at once — not git worktrees, which parallel: true creates
under branch:dev just as well. The old name also echoed the very value it
sets (branch: worktree).

Author: TitusKirch

.tituskirch-skills.json

@@ -108,7 +108,7 @@
     ]
   },
   "profiles": {
-    "worktree": {
+    "fleet": {
       "work": {
         "concurrency": 8,
         "branch": "worktree",