Fix concore validate exit code and add missing source file checks

[Titas-Ghosh]

Hi @pradeeban , This PR fixes issue #303

Summary

• concore validate now exits non‑zero when errors are found.
• Validates node source file existence relative to the workflow (default src/).
• Adds a regression test for missing node source files.
• Documents the --source option for validate.

Problem
concore validate prints errors but still exits with code 0, which makes it ineffective in CI/pre‑commit usage. The README also states file references are validated, but validate didn’t actually check whether node source files exist.

Solution

• validate_workflow now returns success/failure and the CLI exits with code 1 on errors.
• Missing node source files are flagged as errors (checked relative to workflow + source dir).
• Added --source to concore validate to align with run and inspect.

Tests

python -m pytest -v tests/test_cli.py

Checklist

• Tested locally (python -m pytest -v tests/test_cli.py)
• No breaking changes to existing CLI usage
• Docs updated

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Note to Maintainers
Thanks for reviewing! I aimed to align concore validate with the documented behavior and make it reliable for CI by returning a proper exit code and checking missing node source files. If you want the validation scope or error/warning thresholds adjusted, I’m happy to iterate quickly.