Add native Telegram Agent Channel

[mimeding]

Summary

• Adds a native Telegram Agent Channel adapter with token storage, chat allowlists, read/search/send routing, and explicit send confirmation.
• Normalizes webhook and long-poll updates into the shared Agent Channel message store after authorization, with update-id dedupe and cursor tracking.
• Rebased onto the merged Agent Channel JSON runner and inbox/audit foundations; the native Telegram route now coexists with custom JSON channel execution instead of shadowing it.
• Documents current service boundaries: production HTTP receiver, background poller, and visible settings surface remain follow-up integration work.

Validation

• git diff --check
• swiftlint lint Packages/OsaurusCore/Models/AgentChannel/AgentChannelConfiguration.swift Packages/OsaurusCore/Models/Telegram/TelegramConnectionConfiguration.swift Packages/OsaurusCore/Services/AgentChannel/AgentChannelConnectionManager.swift Packages/OsaurusCore/Services/AgentChannel/AgentChannelConnectionService.swift Packages/OsaurusCore/Services/Telegram/TelegramAPIClient.swift Packages/OsaurusCore/Services/Telegram/TelegramConnectionService.swift Packages/OsaurusCore/Tests/Telegram/TelegramConnectionTests.swift
• OSAURUS_DISABLE_KEYCHAIN_FOR_TESTS=1 OSAURUS_TEST_ROOT=/tmp/osaurus-test-telegram swift test --package-path Packages/OsaurusCore --filter TelegramConnectionTests
• OSAURUS_DISABLE_KEYCHAIN_FOR_TESTS=1 OSAURUS_TEST_ROOT=/tmp/osaurus-test-agent-channel swift test --package-path Packages/OsaurusCore --filter AgentChannelAsyncSubstrateTests
• OSAURUS_DISABLE_KEYCHAIN_FOR_TESTS=1 OSAURUS_TEST_ROOT=/tmp/osaurus-test-telegram-final-2 swift test --package-path Packages/OsaurusCore --filter TelegramConnectionTests

Draft gate

• Kept draft intentionally while GitHub CI reruns on the rebased branch.
• Keep draft until the final public channel registration/UI placement is confirmed.
• No agent-loop prompt/tool-schema behavior changes are included, so no eval evidence is required for this service-level adapter refresh.

[mimeding]

Closing this as superseded by #1808. The consolidated #1808 PR includes the Telegram Agent Channel work from this branch plus the Slack/product polish and is the only Agent Channels review/merge target. Please do not merge this older split PR separately.