[codex] Fix local agent discovery imports

[brandonkachen]

Summary

Fixes local agent discovery so loading .agents no longer recurses into known non-agent/support directories that can contain executable scripts.

Root Cause

The SDK local agent loader recursively imported every JS/TS-like file under .agents except skills/. That meant unrelated executable scripts, including quarantined skill scripts under directories like skills-quarantine, could run top-level side effects while Freebuff or Codebuff was only trying to discover local agents.

Changes

• Skip hidden directories, node_modules, scripts, skills, and skills-* during local agent discovery.
• Preserve the existing behavior for candidate agent files outside those directories instead of using source-level heuristics.
• Add regression tests proving quarantined skill scripts and support scripts are not imported.
• Add coverage showing shorthand agent exports still load.

Validation

• bun test sdk/src/__tests__/load-agents.test.ts
• bun run --cwd sdk typecheck
• git diff --check