feat: add reposition method to snacks terminal provider

[3dyuval]

Summary

• Adds M.reposition() to lua/claudecode/terminal/snacks.lua that closes the current window (without killing the terminal process) and reopens the existing buffer in a new Snacks.win with different position opts
• Exposes M.reposition(opts_override) on the public lua/claudecode/terminal.lua module, delegating to the provider if supported
• Snacks-only: the call is guarded by if type(provider.reposition) == "function" so native/external providers are unaffected

Usage

local base_win_opts = opts.terminal.snacks_win_opts
local is_float = true
vim.api.nvim_create_user_command("ClaudeTogglePosition", function()
  local term = require("claudecode.terminal")
  if is_float then
    term.reposition({ snacks_win_opts = vim.tbl_extend("force", base_win_opts, { position = "right", width = 0.4 }) })
  else
    term.reposition({ snacks_win_opts = base_win_opts })
  end
  is_float = not is_float
end, {})

Closes #225