fix: correct buffer parser and timestamp test

Problem: parse_buffer classified /id/-prefixed task lines as headers
because '/' matches the '^%S' header pattern. Store timestamp test
was flaky when add and update ran within the same second.

Solution: check for task line patterns (id prefix or 2-space indent)
before falling through to the header branch. Backdate the initial
modified timestamp in the store update test.

spec/store_spec.lua

@@ -106,11 +106,11 @@ describe('store', function()
     it('updates fields and sets modified', function()
       store.load()
       local t = store.add({ description = 'Original' })
-      local original_modified = t.modified
+      t.modified = '2025-01-01T00:00:00Z'
       store.update(t.id, { description = 'Updated' })
       local updated = store.get(t.id)
       assert.are.equal('Updated', updated.description)
-      assert.is_not.equal(original_modified, updated.modified)
+      assert.is_not.equal('2025-01-01T00:00:00Z', updated.modified)
     end)
 
     it('sets end timestamp on completion', function()