feat: fire User autocmds for plugin state changes

[3dyuval]

Summary

Fires doautocmd User <event> at key plugin state transitions so configs and other plugins can react without poking at internals.

Event	When
ClaudeCodeDiffOpened	A diff view is created (_open_native_diff)
ClaudeCodeDiffClosed	A diff is accepted (_resolve_diff_as_saved) or rejected (_resolve_diff_as_rejected)
ClaudeCodeConnected	A WebSocket client connects (on_connect)
ClaudeCodeDisconnected	A WebSocket client disconnects (on_disconnect)
ClaudeCodeTerminalOpened	Terminal window becomes visible (both snacks and native providers)
ClaudeCodeTerminalHidden	Terminal window is hidden (both snacks and native providers)

Usage

Auto-hide the terminal float when a diff opens, so the diff buffers aren't covered:

vim.api.nvim_create_autocmd("User", {
  pattern = "ClaudeCodeDiffOpened",
  callback = function()
    local term = require("claudecode.terminal")
    if term.get_active_terminal_bufnr() then
      term.simple_toggle()
    end
  end,
})

Changes

• lua/claudecode/diff.lua: ClaudeCodeDiffOpened before returning from _open_native_diff; ClaudeCodeDiffClosed in both _resolve_diff_as_saved and _resolve_diff_as_rejected
• lua/claudecode/server/init.lua: ClaudeCodeConnected / ClaudeCodeDisconnected in the WebSocket callbacks, wrapped in vim.schedule since callbacks run off the main thread
• lua/claudecode/terminal/snacks.lua: ClaudeCodeTerminalOpened / ClaudeCodeTerminalHidden in open, simple_toggle, and focus_toggle
• lua/claudecode/terminal/native.lua: same events in open_terminal, hide_terminal, and show_hidden_terminal

Closes #222