[Refactor] Generalize Fast conversation boundaries

[daniel-lxs]

What

• introduce provider-neutral Fast conversation, reply, reaction, and turn-adapter types
• make the Fast service, session lookup, turn locking, and integration broker consume one conversation identity
• keep Slack and Discord delivery details in their surface adapters
• preserve existing persisted and Redis identities through an explicit compatibility mapping

Why

Fast already behaves as one conversational orchestrator across human messages and delegated-task updates, but its shared service boundary was still expressed in Slack-specific coordinates and callbacks. This refactor makes that boundary match the product model and gives later lifecycle-delivery work a clean provider-neutral core to build on.

This is intentionally behavior-preserving. The existing Slack-shaped persistence schema and Slack task-event parent remain behind adapters; generalizing lifecycle parent delivery is a separate follow-up.

Validation

• pnpm --filter @roomote/cloud-agents check-types
• pnpm --filter @roomote/api check-types
• pnpm --filter @roomote/sdk check-types
• full @roomote/cloud-agents Vitest suite
• full @roomote/api Vitest suite (206 files, 1,786 tests)
• full @roomote/sdk Vitest suite (99 files, 995 tests)
• pnpm lint:fast
• pnpm check-types:fast
• pnpm knip

[roomote-community]

No code issues found. See task

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