docs: clarify shared runner's execution provenance boundary

[dinakarjs]

The shared runner accepts basis="independent_run" with independent_live_reproduction=true without authenticating who executed the run. Imran ruled in upstream agentrust-io#288 that this flag is the reporter's own claim, and requested a contract clarification: agentrust-io#288 (review).

This two-file follow-up implements that ruling in README.md and the existing native_provenance description. It explains that case validation does not establish independent execution, keeps the current vendor adapters at author_record / false, and leaves any future evidence-backed provenance-appraisal mechanism to separate review. No runtime or schema change is included.

Validation at a472a1e3dc01d6a79b9ba1dc8cbcb7bc8eadb048: all eight workflows pass, including Lint and Alakris authority comparison evidence checks. The evidence workflow fetched 20 immutable artifacts, validated 20 reference shapes, reported package checks PASS, MintID bytes PASS, corrected Proofable bytes/envelopes PASS, historical checksum FAIL preserved, and independent comparison incomplete. Evidence: https://github.com/dinakarjs/integrations/actions/runs/38279889516. The earlier link-check HTTP 503 passed on the recorded second attempt; no publisher hashes were changed.

A bounded local reproduction at the unchanged runner head accepted complete author-record reports with a false flag, rejected author-record reports with a true flag, and accepted independent-run reports with a true flag and no provenance. All controls left their inputs unchanged. No native vendor execution was performed.

On October 13, upstream agentrust-io#288 still points to this patch's exact base 74fc43c863709e9e5fb768741a38b35c4c2cdbd8. Imran has approved that base; Carlos's re-review remains requested by October 16. This ready-for-review contributor-fork PR provides the isolated diff for the shared-runner author to integrate. It does not change the upstream branch or merge agentrust-io#288.

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