fix(diff): open the legacy diff's temp file without a swap file

.agents/skills/verify-diff-lifecycle/covered.lock

@@ -3,7 +3,7 @@ file lua/claudecode/config.lua b4eaec92b6d04b40
 == lua/claudecode/cwd.lua
 file lua/claudecode/cwd.lua 55d26ea362173afa
 == lua/claudecode/diff.lua
-file lua/claudecode/diff.lua 53c0f9034a7df662
+file lua/claudecode/diff.lua a21129cdc802e417
 block resolve_saved          46d5f3f732711fe1
 block resolve_rejected       c7a620816abff9a1
 block open_blocking          92067df7f9d4d4d9

lua/claudecode/diff.lua

@@ -813,7 +813,8 @@ function M._open_native_diff(old_file_path, new_file_path, new_file_contents, ta
   vim.cmd("edit " .. vim.fn.fnameescape(old_file_path))
   vim.cmd("diffthis")
   create_split()
-  vim.cmd("edit " .. vim.fn.fnameescape(tmp_file))
+  -- Without a swap file, the rename below cannot meet another Neovim's swap file for "<path> (New)" (E325).
+  vim.cmd("noswapfile edit " .. vim.fn.fnameescape(tmp_file))
   vim.api.nvim_buf_set_name(0, new_file_path .. " (New)")
 
   -- Propagate filetype to the proposed buffer for proper syntax highlighting (#20)

tests/integration/diff_legacy_swap_spec.lua

@@ -0,0 +1,66 @@
+-- Real-Neovim check that the legacy diff.open_diff opens its proposal when another Neovim left a swap file for the
+-- proposal's name ("<path> (New)"). Swap files need a real Neovim, so under busted's mock vim this is pending;
+-- scripts/run_integration_tests_individually.sh runs it in Neovim.
+if vim == nil or vim.fn.has("nvim") ~= 1 then
+  describe("legacy open_diff and a swap file for its proposal", function()
+    pending("needs a real Neovim")
+  end)
+  return
+end
+
+describe("legacy open_diff and a swap file for its proposal", function()
+  local diff = require("claudecode.diff")
+  local dir, swap_dir, path, saved_directory
+
+  before_each(function()
+    dir = vim.fn.tempname()
+    swap_dir = dir .. "/swap"
+    vim.fn.mkdir(swap_dir, "p")
+    path = dir .. "/foo.lua"
+    vim.fn.writefile({ "local a = 1" }, path)
+    saved_directory = vim.o.directory
+    vim.o.directory = swap_dir .. "//"
+    diff.setup({ diff_opts = {} })
+  end)
+
+  after_each(function()
+    vim.cmd("silent! diffoff!")
+    vim.cmd("silent! only")
+    vim.cmd("silent! %bwipeout!")
+    vim.o.directory = saved_directory
+    vim.fn.delete(dir, "rf")
+  end)
+
+  it("opens the proposal when a killed Neovim left a swap file for '<path> (New)'", function()
+    -- Another Neovim changes a buffer named "<path> (New)", so its swap file records that name, and is killed.
+    vim.fn.system({
+      vim.v.progpath,
+      "--headless",
+      "-u",
+      "NONE",
+      "-i",
+      "NONE",
+      "--cmd",
+      "set directory=" .. vim.fn.fnameescape(swap_dir) .. "//",
+      "-c",
+      "edit " .. vim.fn.fnameescape(path .. " (New)"),
+      "-c",
+      "call setline(1, 'unsaved')",
+      "-c",
+      "preserve",
+      "-c",
+      "lua (vim.uv or vim.loop).kill(vim.fn.getpid(), 9)",
+    })
+    assert.are.equal(1, #vim.fn.glob(swap_dir .. "/*", false, true), "the killed Neovim left its swap file")
+
+    vim.cmd("edit " .. vim.fn.fnameescape(path))
+    local ok, result = pcall(diff.open_diff, path, path, "local a = 2\n", "legacy swap")
+
+    assert.is_true(ok, tostring(result))
+    assert.is_true(result.success)
+    local proposal = vim.api.nvim_get_current_buf()
+    assert.are.equal(path .. " (New)", vim.api.nvim_buf_get_name(proposal))
+    assert.is_false(vim.bo[proposal].swapfile)
+    assert.are.equal("", vim.fn.swapname(proposal))
+  end)
+end)