fix: wire adapter mcp_setup() call in session start

The adapter setup() wrote persona files but mcp_setup() was never
called — Gemini .gemini/settings.json and OpenCode opencode.json
were not written to the session directory. Found by running the
multi-agent integration tests with real tmux.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: taylaand

packages/studyctl/src/studyctl/cli/_study.py

@@ -372,10 +372,12 @@ def _handle_start(
     if backlog_notes:
         previous_notes = f"{previous_notes}\n\n{backlog_notes}" if previous_notes else backlog_notes
 
-    # Build persona via adapter pattern
+    # Build persona + MCP config via adapter pattern
     adapter = AGENTS[agent]
     canonical = build_canonical_persona(mode, topic, energy, previous_notes=previous_notes)
     persona_file = adapter.setup(canonical, session_dir)
+    if adapter.mcp_setup:
+        adapter.mcp_setup(session_dir)
 
     # Allow integration tests to inject a mock agent command
     import os