Defer start_polling() till VimEnter

[ShayHill]

Wrap claude_code#diff#start_polling() in a VimEnter augroup to allow Vim to set shellxquote before calling git.

Patch: failure to execute claude_code#diff#start_polling().

This bug likely requires installing the plugin to the Vim8 pack directory.

• $MYVIMDIR\pack\...\start\vim-claude-code
• set g:claude_code_diff_preview = 1

s:dispatch_preview runs before VimEnter. When g:claude_code_diff_preview = 1, s:dispatch_preview calls claude_code#diff#start_polling(), which eventually calls git, which fails because shellxquote is empty while the vimrc is parsed. Vim sets shellxquote before VimEnter so, with the patch, the call to git happens (and succeeds) after shellxquote is set.