feat(ui): redesign mission feed

[yicheng47]

Summary

• render mission events as grouped chat-style avatar rows with goal dividers and expandable signal lines
• add deterministic runner identicons, per-handle hues, ask-card identity, and rail presence overlays
• preserve the inherited bug: markdown list markers render outside the message block in the feed #392 message-body spacing/list fixes
• include the separately scoped feat: redesign runner & crew detail pages #393 runner and crew detail redesign plan as requested

Validation

• pnpm exec tsc --noEmit
• pnpm run lint
• pnpm test (318 passed)
• git diff --check
• manual smoke test passed

Review

• working-tree review completed before commit
• final verdict clean with no remaining must-fix issues