feat(a2a): add external evidence references

[pshkv]

Summary

• add optional A2A Agent Card external evidence references for authority, tool-surface, signed-tool-definition, counterparty-safety, and verification-state records
• add helper functions to filter external evidence by type, subject, and freshness without treating those records as Agent Card identity
• cover registry preservation, freshness filtering, and stale evidence behavior in bridge-a2a tests

Context

This follows the A2A Agent Card identity thread: keep card identity stable while authority and counterparty/tool-surface safety ride as separately verifiable, freshness-bounded evidence.

Validation

• pnpm --filter @pshkv/bridge-a2a typecheck
• pnpm --filter @pshkv/bridge-a2a test
• pnpm --filter @pshkv/bridge-a2a build
• pnpm --filter @pshkv/conformance-tests test -- src/a2a-fixtures-conformance.test.ts src/autogen-interop-conformance.test.ts
• git diff --check