feat(a2a): add secure interaction test kit

[thinksyncs]

Summary

• Add an executable A2A security interaction test kit and selected LLM-to-LLM workflow.
• Add the experimental draft06-v2 Accepted Assertion path and reproducible wire evidence.

Changes

• Add strict reports, response validation, multi-host role bundles, and linked run evidence.
• Add a shared TLS Redis/Valkey acceptance store and durable at-most-once operation journal.
• Store successful A2A responses as encrypted envelopes for authenticated exact-response recovery.
• Retain replay through the later proof or challenge expiry and reject ambiguous JSON.

Testing

• go test -count=1 ./...
• Focused go test -race -count=1 for identity, schemas, reports, journal, Redis, and multiprocess paths.
• go vet ./...
• Critical boundary tests repeated 20 times; Redis transaction tests repeated 5 times.
• Python context and full-wire verifiers, including three ES256 signatures.

Notes

• The journal provides at-most-once invocation, not exactly-once external effects or response delivery.
• Real Redis failover, physical multi-host execution, and independent-vendor interoperability were not run.
• The Redis adapter requires writable-primary routing; WAIT is not a zero-loss guarantee.