fix(feed): sanitize multiline previews to prevent overlay layout corruption

[monotykamary]

Summary

Before	After

Fixes overlay layout corruption caused by multiline data in .pi/messenger/feed.jsonl.

The root cause was feed events with embedded newlines (for example, commit/message previews). Those newline characters were rendered into the TUI row pipeline and broke panel borders/section alignment.

What changed

• feed.ts
  • Added feed event sanitization for inline text fields (agent, target, preview)
  • Applied sanitization when appending events, reading events, and formatting feed lines
• overlay-render.ts
  • Sanitizes feed events before rendering message and non-message lines
• overlay.ts
  • Hardened row rendering by stripping control whitespace (\n, \r, \t) and truncating to section width before drawing borders
• Tests
  • Added regression coverage for multiline preview normalization and rendering safety:
    • tests/feed.test.ts
    • tests/crew/live-feed.test.ts

Validation

• npm test passes (full suite)
• Reproduced problematic .pi data now renders cleanly without layout breakage