Fix session picker module import and formatting

[nakhan98]

Summary

• change the session picker to require opencode.api instead of the nonexistent lua.opencode.api
• ensure vim.ui.select receives strings so its formatter no longer raises a concat error

Environment

• vim-plug used for plugin management
• Local checkout via Plug '~/dev/github/opencode.nvim' in init.vim
• macOS 15.7.2 with Neovim 0.11.5
• Opencode version 1.0.65

Snippets

-- lua/opencode/ui/session_picker.lua
-local api = require('lua.opencode.api')
+local api = require('opencode.api')

-- lua/opencode/ui/ui.lua
-local parts = { { session.id } }
+local parts = { session.id }

Error Logs

Error executing Lua callback: .../lua/opencode/ui/session_picker.lua:5: module 'lua.opencode.api' not found
  no file './lua/opencode/api.lua' ...
  ?
Error executing Lua callback: .../lua/vim/ui.lua:48: invalid value (table) at index 1 in table for 'concat'
Stack traceback: ...

Testing

• not run (per agent instructions)

[cameronr]

First issue fixed in #120, second one in #123