feat: publish bounded runtime execution evidence

[Pigbibi]

Summary

• add a reusable composite Action for existing platform heartbeat workflows to publish a bounded, read-only runtime-evidence snapshot
• reuse each platform's existing GitHub OIDC authentication; no GCP service-account key is created or accepted
• preserve the oldest accepted runtime report timestamp and reject stale/future reports so recollection cannot make stale evidence appear current

Safety boundary

• lists at most 100 recent JSON reports only under the caller's configured platform prefix
• copies no summary, diagnostics, artifacts, runtime error text, account data, positions, orders, fills, capital data, credentials, or object URLs into the snapshot
• target data/execution remain pending; recommendation remains parked
• action does not change strategies, runtime targets, brokers, or orders

Validation

• python3 -m unittest python/tests/test_execution_evidence_projection.py (5 passed)
• python3 -m py_compile python/scripts/execution_evidence_projection.py
• git diff --check
• actionlint and full CI run on GitHub.