chore: add SPDX-License-Identifier headers to Python scripts

[leadsolutions-mike]

Summary

Closes #7.

Adds # SPDX-License-Identifier: Apache-2.0 to the five Python scripts in .agents/scripts/ that were missing the header:

• validate_state.py — after the shebang line
• migrate_to_v2.py — after the shebang line
• test_validate_state.py — at the top of the file
• test_migrate_to_v2.py — at the top of the file
• conftest.py — at the top of the file

No functional changes. This brings the scripts into compliance with the Apache 2.0 license declared in LICENSE and makes them compatible with SPDX-aware license scanners (FOSSA, Snyk License, etc.).

Test plan

• CI passes (state-validation workflow)
• Verify SPDX header is present in each of the 5 files
• Confirm no functional change: pytest .agents/scripts/ -v produces the same results as before

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Contributed by @leadsolutions-mike as external collaborator.

[mmilanez]

Reviewed the diff and validation results. The PR only adds SPDX-License-Identifier headers to the expected Python scripts, satisfies #7, and both required checks pass.