fix: resolve selene lint warnings and errors

Problem: unused variables (undo_state, sha_result, date match
captures) and duplicate if/else branches in gcal sync triggered
selene warnings and errors.

Solution: prefix unused state with underscore, simplify date
validation to a single pattern match, remove dead sha_result
call, and merge duplicate event deletion branches into a single
condition.

spec/parse_spec.lua

@@ -103,7 +103,7 @@ describe('parse', function()
     end)
 
     it('ignores lowercase prefix', function()
-      local desc, meta = parse.command_add('hello: world')
+      local desc, _ = parse.command_add('hello: world')
       assert.are.equal('hello: world', desc)
     end)