fix(tui): harden collapse.lines against non-string content

[YOMXXX]

Summary

Fixes #1952. TypeError: A.replace is not a function (minified A = content) when collapse.lines receives a truthy non-string value.

Root cause: packages/opencode/src/cli/cmd/tui/util/collapse.ts lines() guard if (!content) return [] only catches falsy values — a truthy non-string (number/object) passes and content.replace is undefined → crash. Tool output / patch content rendered in the TUI can be a non-string under unusual model output.

Fix: tighten the guard to reject any non-string while preserving the empty-string → [] behavior (which rows("") depends on):

export function lines(content: unknown) {
  if (typeof content !== "string" || content === "") return []
  return content.replace(/\n$/, "").split("\n")
}

Test Plan

• New collapse.lines tests: non-string truthy → []; undefined/null/"" → []; strings still split with trailing newline stripped.
• bun test test/cli/tui/collapse.test.ts — 19 pass.
• bun typecheck — clean.