feat(complete): add metadata completion for :Pending add (#144)

Problem: `:Pending add` had no tab completion for inline metadata
tokens, unlike `:Pending edit` which already completed `due:`, `rec:`,
and `cat:` values.

Solution: Add `complete_add()` that handles `due:`, `rec:`, and `cat:`
prefix matching with the same value sources used by `complete_edit()`,
and wire it into the command completion dispatcher.

plugin/pending.lua

@@ -80,6 +80,65 @@ local function filter_candidates(lead, candidates)
   end, candidates)
 end
 
+---@param arg_lead string
+---@return string[]
+local function complete_add(arg_lead)
+  local cfg = require('pending.config').get()
+  local dk = cfg.date_syntax or 'due'
+  local rk = cfg.recur_syntax or 'rec'
+  local ck = cfg.category_syntax or 'cat'
+
+  local prefix = arg_lead:match('^(' .. vim.pesc(dk) .. ':)(.*)$')
+  if prefix then
+    local after_colon = arg_lead:sub(#prefix + 1)
+    local result = {}
+    for _, d in ipairs(edit_date_values()) do
+      if d:find(after_colon, 1, true) == 1 then
+        table.insert(result, prefix .. d)
+      end
+    end
+    return result
+  end
+
+  local rec_prefix = arg_lead:match('^(' .. vim.pesc(rk) .. ':)(.*)$')
+  if rec_prefix then
+    local after_colon = arg_lead:sub(#rec_prefix + 1)
+    local result = {}
+    for _, p in ipairs(edit_recur_values()) do
+      if p:find(after_colon, 1, true) == 1 then
+        table.insert(result, rec_prefix .. p)
+      end
+    end
+    return result
+  end
+
+  local cat_prefix = arg_lead:match('^(' .. vim.pesc(ck) .. ':)(.*)$')
+  if cat_prefix then
+    local after_colon = arg_lead:sub(#cat_prefix + 1)
+    local store = require('pending.store')
+    local s = store.new(store.resolve_path())
+    s:load()
+    local seen = {}
+    local cats = {}
+    for _, task in ipairs(s:active_tasks()) do
+      if task.category and not seen[task.category] then
+        seen[task.category] = true
+        table.insert(cats, task.category)
+      end
+    end
+    table.sort(cats)
+    local result = {}
+    for _, c in ipairs(cats) do
+      if c:find(after_colon, 1, true) == 1 then
+        table.insert(result, cat_prefix .. c)
+      end
+    end
+    return result
+  end
+
+  return {}
+end
+
 ---@param arg_lead string
 ---@param cmd_line string
 ---@return string[]
@@ -207,6 +266,9 @@ end, {
       end
       return filtered
     end
+    if cmd_line:match('^Pending%s+add%s') then
+      return complete_add(arg_lead)
+    end
     if cmd_line:match('^Pending%s+archive%s') then
       return filter_candidates(arg_lead, { '7d', '2w', '30d', '3m', '6m', '1y' })
     end