Fix MCP tool availability and shell escaping in tag mode

Pass MCP config and allowed tools through claude_args to ensure tools like
mcp__github_comment__update_claude_comment are properly available to Claude CLI.

Key changes:
- Tag mode outputs claude_args with MCP config (as JSON string) and allowed tools
- Fixed shell escaping vulnerability when JSON contains single quotes
- Agent mode passes through user-provided claude_args unchanged
- Re-added mcp_config input for users to provide custom MCP servers
- Cleaned up misleading comments and unused file operations
- Clarified test workflow is for fork testing

Security fix: Properly escape single quotes in MCP config JSON to prevent
shell injection vulnerabilities.

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

src/modes/agent/index.ts

@@ -79,7 +79,9 @@ export const agentMode: Mode = {
       }
     }
 
-    core.setOutput("mcp_config", JSON.stringify(mcpConfig));
+    // Agent mode: pass through user's claude_args without modification
+    const userClaudeArgs = process.env.CLAUDE_ARGS || "";
+    core.setOutput("claude_args", userClaudeArgs);
 
     return {
       commentId: undefined,