fix(github): reject garbage in _parse_date; consolidate date parsing

github.py _parse_date used naive string slicing (return iso_str[:10])
which accepted any 10+ character string as a "date." For input
"hello world" it returned "hello worl". Now delegates to
dates.parse_date() which validates the format and returns None for
non-dates.

Also migrated reddit.py and threads.py _parse_date to the shared
dates.parse_date(). Both previously reimplemented ISO-with-trailing-
offset handling (the .replace("Z", "+00:00") dance) and reddit.py
also had its own Unix timestamp branch. dates.parse_date() already
handles all of this, including the +0000 no-colon variant Reddit emits.

Preserved reddit.py's original falsy-check so 0 still returns None
(epoch 0 would otherwise parse as "1970-01-01", breaking an existing
test and changing long-standing behavior).

Added 4 new github tests for garbage rejection and offset variants.
All 1026 existing tests pass (15 pre-existing failures unchanged).

tests/test_github.py

@@ -56,6 +56,22 @@ class TestParseDate(unittest.TestCase):
     def test_empty(self):
         self.assertIsNone(github._parse_date(""))
 
+    def test_rejects_garbage(self):
+        """The old naive slicing returned 'hello worl' for 'hello world'. Reject it."""
+        self.assertIsNone(github._parse_date("hello world"))
+        self.assertIsNone(github._parse_date("not-a-date"))
+        self.assertIsNone(github._parse_date("abcdefghij"))
+
+    def test_rejects_invalid_date_values(self):
+        """An out-of-range date like 2026-99-99 is not a real date."""
+        self.assertIsNone(github._parse_date("2026-99-99"))
+
+    def test_iso_with_offset(self):
+        self.assertEqual(github._parse_date("2026-03-15T12:00:00+00:00"), "2026-03-15")
+
+    def test_iso_with_no_colon_offset(self):
+        self.assertEqual(github._parse_date("2026-03-15T12:00:00+0000"), "2026-03-15")
+
 
 class TestSearchGithub(unittest.TestCase):
     @patch.dict("os.environ", {}, clear=True)