fix: remove before_session hook startup color

[christso]

Summary

• remove yellow ANSI styling from the before_session startup log line
• add a regression test that runs the hook and asserts the startup log is plain text

Red/Green evidence

Red (origin/main)

"\u001b[33mRunning before_session hook: bun -e \"process.stdout.write('HOOK_VAR=ok\\n')\"\u001b[0m"

Green (this branch)

"Running before_session hook: bun -e \"process.stdout.write('HOOK_VAR=ok\\n')\""

Test Plan

• bun test packages/core/test/evaluation/hooks.test.ts
• bun run build
• git push -u origin fix/before-session-hook-color (pre-push hooks)

[cloudflare-workers-and-pages]

Deploying agentv with    Cloudflare Pages

Latest commit:	1797bec
Status:	✅  Deploy successful!
Preview URL:	https://06d11675.agentv.pages.dev
Branch Preview URL:	https://fix-before-session-hook-colo.agentv.pages.dev

View logs