fix: validate submit_pr required arguments

[cybercraftsolutionsllc]

Fixes #15.

Summary:

• validate submit_pr required string args before posting to /submissions
• return clear field-specific MCP errors for missing or blank task_id, agent_id, result_text, and external_link
• add a small injectable submit helper so the regression test can assert no network sender is called on invalid input
• add a node:test regression suite and refresh vulnerable transitive lockfile entries within the existing semver range

Validation:

• npm test
• npm audit --omit=dev --json
• git diff --check

Payout route if needed: 0xB34D185318b34ec2F9E060F662Cc7feA3180049c