Validation errors leak absolute config file paths to user

[jariy17]

Summary

When Zod validation fails on agentcore.json, error messages include the absolute filesystem path to the config file. This leaks internal directory structure to the user.

Reproducer

agentcore add memory --name "bad name!" --expiry 30 --json

Actual behavior

{"success":false,"error":"/Users/username/path/to/project/agentcore/agentcore.json:\n  - memories[1].name: Must begin with a letter..."}

Expected behavior

{"success":false,"error":"agentcore.json:\n  - memories[1].name: Must begin with a letter..."}

Or just the error message without the file path.

Notes

Affects all add and remove commands that go through ConfigIO.writeProjectSpec → validateAndWrite. The path comes from ConfigValidationError in config-io.ts.