Customize Slack in-flight response status

[cyrusagent]

Summary

Implements Cyrus-branded Slack in-flight status for chat sessions and adds Slack stream helpers that keep each stream in a single mode.

• Sets assistant.threads.setStatus with Cyrus-specific status/loading copy when a Slack turn starts or receives a streamed follow-up.
• Clears the active Slack status on successful result handling and on runner start/resume errors.
• Adds Slack chat.startStream / chat.appendStream / chat.stopStream helpers with runtime guardrails for markdown-vs-chunks mode mixing and required channel recipient fields.
• Removes prototype presentation from production behavior by omitting username/icon overrides by default so Slack uses the installed Cyrus app identity.

Validation

• pnpm build
• pnpm typecheck
• pnpm test:packages:run
• pnpm lint
• pnpm --dir packages/slack-event-transport exec vitest run test/SlackMessageService.test.ts
• pnpm --dir packages/edge-worker exec vitest run test/chat-sessions.test.ts
• F1 synthetic Slack chat smoke documented in apps/f1/test-drives/2026-07-03-cypack-1367-slack-inflight-status.md

Notes

F1 validates the Slack chat handler path but cannot visually confirm Slack-rendered assistant status because it uses synthetic channels. A live Slack thread remains the final manual presentation check.

Linear: https://linear.app/ceedar/issue/CYPACK-1367/customize-cyrus-slack-in-flight-response-status-and-streaming

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