fix(fleet): remove duplicate workingDirectory key in roster.schema.json

Edit artifact from the host/ssh addition left a third workingDirectory block in
the agent-items properties (duplicate JSON keys are last-wins, so prettier/CI did
not flag it). Keep exactly one workingDirectory + one working_directory.

Refs #620

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EsgTQzV5YUGk1JtCLP4B83

packages/mosaic/framework/fleet/roster.schema.json

@@ -99,9 +99,6 @@
           "workingDirectory": {
             "type": "string"
           },
-          "workingDirectory": {
-            "type": "string"
-          },
           "model_hint": {
             "type": "string"
           },