Add channel remote safety gate

[mimeding]

Summary

Adds a provider-neutral remote safety layer for Agent Channels before live Discord, Slack, Telegram, or JSON adapters dispatch into an agent loop.

What changed

• Added ChannelRemoteSafetyPolicy and ChannelRemoteSafetyGate.shared for remote receive/reply/Computer Use handoff flows.
• Added a service-produced ChannelVerifiedReplyTokenValidation wrapper so remote safety requests use reply-token validations produced by ChannelReplyTokenService.
• Re-checks accepted token payload identity, purpose, action, write-gate generation, issue time, expiry, and in-process replay state before allowing dangerous remote actions.
• Requires stable task ids for remote Computer Use starts, limits active tasks per channel identity, supports explicit task finish, and prunes stale in-memory state.
• Rate-limits authorized remote actions per normalized channel identity without burning accepted proof on later rate/task denials.
• Wraps inbound channel text as untrusted JSON-framed content and sanitizes channel-returned result text so credentials, reply tokens, and oversized payloads are not echoed into shared rooms.
• Updates channel docs with the required token-service-then-gate integration order and makes clear this helper is opt-in until adapters call it.

Validation

• git diff --check
• swiftlint lint --quiet Packages/OsaurusCore/Models/Channels/ChannelRemoteSafetyPolicy.swift Packages/OsaurusCore/Services/Channels/ChannelRemoteSafetyGate.swift Packages/OsaurusCore/Services/Channels/ChannelReplyTokenService.swift Packages/OsaurusCore/Tests/Channels/ChannelRemoteSafetyGateTests.swift
• OSAURUS_DISABLE_KEYCHAIN_FOR_TESTS=1 OSAURUS_TEST_ROOT=/tmp/osaurus-channel-remote-safety swift test --package-path Packages/OsaurusCore --filter ChannelRemoteSafetyGateTests
• OSAURUS_DISABLE_KEYCHAIN_FOR_TESTS=1 OSAURUS_TEST_ROOT=/tmp/osaurus-channel-security swift test --package-path Packages/OsaurusCore --filter ChannelSecurityTests
• OSAURUS_DISABLE_KEYCHAIN_FOR_TESTS=1 OSAURUS_TEST_ROOT=/tmp/osaurus-channel-substrate swift test --package-path Packages/OsaurusCore --filter AgentChannelAsyncSubstrateTests
• OSAURUS_TEST_ROOT=/tmp/osaurus-channel-remote-safety-keychain swift test --package-path Packages/OsaurusCore --filter AgentManagerLifecycleNotificationTests/deleteSweepsPerAgentKeychainSecrets

Notes

• This is a helper-layer PR. No live adapter is protected until it explicitly invokes ChannelRemoteSafetyGate.shared.
• Full local make test with Keychain disabled hit the expected keychain-specific lifecycle failure; the isolated keychain test passed without the disabled-keychain flag.
• A canonical full local make test pass was started without the disabled-keychain flag but was interrupted after a prolonged local stall, so this draft does not claim full local-suite green evidence yet.