perf: optimize string concatenation and namespace caching

[jinzhongjia]

Summary

• Replace string concatenation in loops with table.concat in ID generation
• Cache namespace ID at module level instead of repeated creation
  Changes
  lua/opencode/id.lua
• random_base62(): Use table + table.concat instead of result = result .. char (O(n) vs O(n²))
• generate_new_id(): Same optimization for timestamp byte extraction
  lua/opencode/ui/input_window.lua
• Cache placeholder_ns at module level
• Remove 3 redundant nvim_create_namespace('input_placeholder') calls
  Why
  String concatenation in Lua creates new string objects each iteration, causing O(n²) complexity. While the impact is minimal for short strings (26 chars), this follows better Lua practices.
  Namespace IDs are stable for the same name, but caching avoids unnecessary function calls on every placeholder refresh.

[jinzhongjia]

This is a little optimize, it will increase our performance a bit

[sudo-tee]

The id calss is rarely used. I think it's only used for the init command.

But I am ok with it being more optimized. It was a loose port from the opencode implementation in typescript

Thanks for the PR