feat(parse): add inline metadata parser

Problem: need to extract due dates and categories from task
descriptions typed in the buffer.

Solution: add right-to-left token parser for configurable date
syntax (default 'due:') and cat: metadata keys. Supports
Category: prefix syntax for :Todo add commands.

lua/todo/parse.lua

@@ -0,0 +1,98 @@
+local config = require('todo.config')
+
+---@class todo.parse
+local M = {}
+
+---@param s string
+---@return boolean
+local function is_valid_date(s)
+  local y, m, d = s:match('^(%d%d%d%d)-(%d%d)-(%d%d)$')
+  if not y then
+    return false
+  end
+  y, m, d = tonumber(y), tonumber(m), tonumber(d)
+  if m < 1 or m > 12 then
+    return false
+  end
+  if d < 1 or d > 31 then
+    return false
+  end
+  local t = os.time({ year = y, month = m, day = d })
+  local check = os.date('*t', t)
+  return check.year == y and check.month == m and check.day == d
+end
+
+---@return string
+local function date_key()
+  return config.get().date_syntax or 'due'
+end
+
+---@param text string
+---@return string description
+---@return { due?: string, cat?: string } metadata
+function M.body(text)
+  local tokens = {}
+  for token in text:gmatch('%S+') do
+    table.insert(tokens, token)
+  end
+
+  local metadata = {}
+  local i = #tokens
+  local dk = date_key()
+  local date_pattern = '^' .. vim.pesc(dk) .. ':(%d%d%d%d%-%d%d%-%d%d)$'
+
+  while i >= 1 do
+    local token = tokens[i]
+    local due_val = token:match(date_pattern)
+    if due_val then
+      if metadata.due then
+        break
+      end
+      if not is_valid_date(due_val) then
+        break
+      end
+      metadata.due = due_val
+      i = i - 1
+    else
+      local cat_val = token:match('^cat:(%S+)$')
+      if cat_val then
+        if metadata.cat then
+          break
+        end
+        metadata.cat = cat_val
+        i = i - 1
+      else
+        break
+      end
+    end
+  end
+
+  local desc_tokens = {}
+  for j = 1, i do
+    table.insert(desc_tokens, tokens[j])
+  end
+  local description = table.concat(desc_tokens, ' ')
+
+  return description, metadata
+end
+
+---@param text string
+---@return string description
+---@return { due?: string, cat?: string } metadata
+function M.command_add(text)
+  local cat_prefix = text:match('^(%S.-):%s')
+  if cat_prefix then
+    local first_char = cat_prefix:sub(1, 1)
+    if first_char == first_char:upper() and first_char ~= first_char:lower() then
+      local rest = text:sub(#cat_prefix + 2):match('^%s*(.+)$')
+      if rest then
+        local desc, meta = M.body(rest)
+        meta.cat = meta.cat or cat_prefix
+        return desc, meta
+      end
+    end
+  end
+  return M.body(text)
+end
+
+return M