refactor: canonicalize internal metadata field names

Problem: `pending.Metadata` used shorthand field names (`cat`, `rec`,
`rec_mode`) matching user-facing token syntax, coupling internal
representation to config. `RecurSpec.from_completion` used a boolean
where a `pending.RecurMode` alias exists. `category_syntax` was
hardcoded to `'cat'` with no config option.

Solution: rename `Metadata` fields to `category`/`recur`/`recur_mode`,
add `category_syntax` config option (default `'cat'`), rename
`ParsedEntry` fields to match, replace `RecurSpec.from_completion`
with `mode: pending.RecurMode`, and restore `[string]` indexer on
`pending.ForgeConfig` alongside explicit fields.

lua/pending/init.lua

@@ -984,10 +984,10 @@ function M.add(text)
   end
   s:add({
     description = description,
-    category = metadata.cat,
+    category = metadata.category,
     due = metadata.due,
-    recur = metadata.rec,
-    recur_mode = metadata.rec_mode,
+    recur = metadata.recur,
+    recur_mode = metadata.recur_mode,
     priority = metadata.priority,
   })
   _save_and_notify()
@@ -1194,6 +1194,7 @@ end
 local function parse_edit_token(token)
   local recur = require('pending.recur')
   local cfg = require('pending.config').get()
+  local ck = cfg.category_syntax or 'cat'
   local dk = cfg.date_syntax or 'due'
   local rk = cfg.recur_syntax or 'rec'
 
@@ -1209,7 +1210,7 @@ local function parse_edit_token(token)
   if token == '-due' or token == '-' .. dk then
     return 'due', vim.NIL, nil
   end
-  if token == '-cat' then
+  if token == '-' .. ck then
     return 'category', vim.NIL, nil
   end
   if token == '-rec' or token == '-' .. rk then
@@ -1231,7 +1232,7 @@ local function parse_edit_token(token)
       'Invalid date: ' .. due_val .. '. Use YYYY-MM-DD, today, tomorrow, +Nd, weekday names, etc.'
   end
 
-  local cat_val = token:match('^cat:(.+)$')
+  local cat_val = token:match('^' .. vim.pesc(ck) .. ':(.+)$')
   if cat_val then
     return 'category', cat_val, nil
   end
@@ -1256,11 +1257,15 @@ local function parse_edit_token(token)
       .. token
       .. '. Valid: '
       .. dk
-      .. ':<date>, cat:<name>, '
+      .. ':<date>, '
+      .. ck
+      .. ':<name>, '
       .. rk
       .. ':<pattern>, +!, -!, -'
       .. dk
-      .. ', -cat, -'
+      .. ', -'
+      .. ck
+      .. ', -'
       .. rk
 end