feat(forge): add gx keymap to open forge links in browser (#165)

Problem: Forge shorthand tokens like `gh:user/repo#42` are concealed in
the buffer so Neovim's built-in `gx` cannot recognize them as URLs.
Full URLs are also concealed, making `gx` unreliable.

Solution: Add `open_link` action (default `gx`) that reads `forge_spans`
from line metadata. If the cursor is on a forge token, that link opens;
otherwise the last ref on the line is used. Delegates to `vim.ui.open()`.

lua/pending/init.lua

@@ -404,6 +404,9 @@ function M._setup_buf_mappings(bufnr)
     edit_notes = function()
       M.open_detail()
     end,
+    open_link = function()
+      M.open_link()
+    end,
   }
 
   for name, fn in pairs(actions) do
@@ -931,6 +934,31 @@ function M.open_detail()
   end, { buffer = detail_bufnr })
 end
 
+---@return nil
+function M.open_link()
+  local bufnr = buffer.bufnr()
+  if not bufnr then
+    return
+  end
+  local row = vim.api.nvim_win_get_cursor(0)[1]
+  local meta = buffer.meta()
+  if not meta[row] or meta[row].type ~= 'task' then
+    return
+  end
+  local spans = meta[row].forge_spans
+  if not spans or #spans == 0 then
+    return
+  end
+  local col = vim.api.nvim_win_get_cursor(0)[2]
+  for _, span in ipairs(spans) do
+    if col >= span.col_start and col < span.col_end then
+      vim.ui.open(span.ref.url)
+      return
+    end
+  end
+  vim.ui.open(spans[#spans].ref.url)
+end
+
 ---@param direction 'up'|'down'
 ---@return nil
 function M.move_task(direction)