docs: update composite strict-or example to bun run

[christso]

Summary

• Update composite grader docs to recommend command-string usage (bun run) for aggregator.path.
• Fix strict-OR section formatting and examples.
• Add runnable strict-OR aggregator example script at examples/features/composite/scripts/or-aggregator.js.
• Add a composite fixture using this script and a dry-run execution command in the README.

Validation

• Executed: bun agentv eval run examples/features/composite/evals/dataset.eval.yaml --test-id strict-or-local --dry-run
• Result artifacts show the script invocation path and strict-OR assertions from the aggregator output.

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