fix: suppress unphased progress messages

[decofe]

Summary

• defer unphased App Server agent messages until their role is known
• discard unphased progress text when a tool or other work event follows
• promote only the terminal unphased message to the final answer
• add regression coverage matching the OpenRouter/Kimi event sequence

Root cause

OpenRouter model moonshotai/kimi-k3 emitted every agentMessage with phase: null. The renderer treated unknown phases as final answers, so pre-tool progress messages were appended to the Slack response.

Validation

• pnpm --filter @centaur/rendering test (34 passed)
• pnpm --filter @centaur/rendering typecheck
• pnpm --filter slackbotv2 test (234 passed, 1 skipped)
• pnpm --filter slackbotv2 run check:types (blocked by existing fetch.preconnect fixture errors in test/slack-user.test.ts)

Prompted by: @Zygimantass