🛡️ Sentinel: Fix CSRF vulnerability by enforcing Origin validation on localhost

[google-labs-jules]

This change removes a security vulnerability where Origin header validation was skipped for requests to localhost or 127.0.0.1. This behavior could allow malicious websites to perform Cross-Site Request Forgery (CSRF) attacks against a locally running MCP server.

Changes:

• Modified src/http-transport.ts to remove the early return that bypassed Origin checks for localhost.
• Updated src/http-transport.test.ts to include a test case verifying that requests with malicious origins are rejected (403 Forbidden) even when the Host is localhost.
• Initialized .jules/sentinel.md with a journal entry documenting this vulnerability and the fix.

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PR created automatically by Jules for task 6786025401319321649 started by @davidruzicka

[google-labs-jules]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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