Add Agent Channel async substrate

[mimeding]

Summary

• Adds provider-neutral async Agent Channel models and services for webhook verification, sender policy, deduplication, reply tokens, session partitioning, audit events, and artifact forwarding status.
• Keeps provider route IDs opaque to agent-visible payloads.
• Adds docs for the async channel contracts and safety boundaries.

Validation

• swift test --package-path Packages/OsaurusCore --filter AgentChannelAsyncSubstrateTests
• swiftlint lint --quiet Packages/OsaurusCore/Models/AgentChannel/AgentChannelAsyncModels.swift Packages/OsaurusCore/Services/AgentChannel/AgentChannelAsyncSubstrate.swift Packages/OsaurusCore/Tests/AgentChannel/AgentChannelAsyncSubstrateTests.swift
• git diff --check
• External review pass completed; actionable findings were incorporated before publication.
• GitHub checks are green.

Notes

• Ready for review as a provider-neutral Agent Channel foundation. This branch does not change default tools, prompts, routing, or model behavior.