fix(sdk): skip plugin code during agent discovery

[lifelmy]

Summary

• exclude .agents/plugins trees from local agent-module discovery
• add a regression test proving a nested MCP CommonJS executable is never imported

Root cause

loadLocalAgents recursively treated every supported JavaScript or TypeScript file under .agents as a candidate agent module and imported it before checking whether it exported an agent definition. Third-party plugin and MCP executables can inspect the host process arguments or call process.exit(), so discovery could terminate the Freebuff CLI.

Closes #1018.

Validation

• bun test sdk/src/__tests__/load-agents.test.ts --test-name-pattern 'does not execute nested MCP implementation files'
• bun test sdk/src/__tests__/load-agents.test.ts (34 passed)
• bun run --cwd sdk typecheck
• bun run build:sdk
• git diff --check