Bound lines array in getLastNVisualLines to prevent unbounded memory growth

[nordicnode]

Bound lines array in getLastNVisualLines to prevent unbounded memory growth

Summary

• In cli/src/utils/text-layout.ts, bound the lines accumulator in getLastNVisualLines to at most n lines.
• Previously, getLastNVisualLines accumulated every wrapped line from character 0 to the end of the text in lines: string[], only to slice lines.slice(-n) at the very end and discard the rest. When rendering long reasoning blocks, assistant messages, or terminal logs (e.g. 500+ lines), this held hundreds of unnecessary line strings in memory and required an additional .slice() array allocation.
• Updated pushLine() to evict the oldest line whenever lines.length === n and set hasMore = true, ensuring memory used by lines is strictly bounded by $O(n)$ ($n \le 5$ in preview) rather than $O(\text{total lines})$.
• Added comprehensive unit tests in cli/src/utils/__tests__/text-layout.test.ts covering zero column/line inputs, small line counts, wrapping boundaries, and multi-hundred line inputs.

Test plan

[✓] bun test --config=/dev/null src/utils/__tests__/text-layout.test.ts — 11 pass, 0 fail
[✓] bun run --cwd cli typecheck — 0 errors
[✓] PR hygiene check passed