fix(diff): user-initiated accept/reject should clean up windows and buffers

[mightreya]

Problem

:ClaudeCodeDiffAccept and :ClaudeCodeDiffDeny (the accept_current_diff / deny_current_diff user commands) leave the diff buffers, split windows, and active_diffs state behind after resolving. Users have to close the leftover windows and buffers by hand.

This shows up as the symptom in #205 and as the partial behavior described in #238 (rejecting via :q closes one buffer but does nothing else).

Root cause

The two user commands call _resolve_diff_as_saved / _resolve_diff_as_rejected, which only set the diff status and resume the MCP coroutine. The actual UI cleanup lives in _cleanup_diff_state and is triggered exclusively by the close_tab MCP tool.

But close_tab has schema = nil and is documented as "Internal tool — must remain as requested by user" (see lua/claudecode/tools/close_tab.lua). The Claude CLI never invokes it. So when a user resolves a diff via the bundled commands, cleanup never fires.

This was less visible before #175 because the cleanup function did less work; with #175 expanding cleanup to close plugin-created splits and force-delete buffers, the gap between "resolve" and "cleanup" became user-facing as accumulating diff windows.

Fix

Have the user commands call _cleanup_diff_state themselves. They're not waiting for an MCP response from the CLI — they're driven by the user — so the "wait for close_tab" architecture doesn't apply to them.

This PR does not touch close_tab.lua. Its schema = nil is left intact per the existing comment.

It also extends diff_opts.keep_terminal_focus to apply at user-resolve time, not only at diff-open time, so focus returns to the Claude terminal after accept/reject.

Tests

Adds tests/unit/diff_user_command_cleanup_spec.lua covering:

• accept_current_diff closes the diff window and removes the entry from active_diffs
• deny_current_diff closes the diff window and removes the entry from active_diffs
• accept_current_diff is a safe no-op when called outside a diff buffer

I was unable to run make test locally (the Nix-based dev shell wasn't available on my machine). I confirmed Lua syntax via loadfile. Please run the suite in CI / locally before merging.

Closes / refs

• Closes [BUG] Diff panels not cleared when new suggestion is shown — stale diffs accumulate until window focus change #205
• Partially addresses [BUG] Rejecting with :q does not work #238