Add dash 4-digit year format and pin year boundary

Dash inputs like 7-3-2026 previously returned None because only the
slash variant had a 4-digit-year format. Add %m-%d-%Y so dash and slash
behave alike, and pin the two-digit-year century boundary with a test.

Refs: #133

src/slack-bot/app.py

@@ -62,6 +62,7 @@ _DATED_FORMATS = (
     "%Y-%m-%d",
     "%m/%d/%Y",
     "%m/%d/%y",
+    "%m-%d-%Y",
     "%m-%d-%y",
     "%b %d %Y",
     "%B %d %Y",

tests/slack_bot/test_parse_date.py

@@ -69,6 +69,19 @@ def test_two_digit_year_dash(slackbot_app):
     assert slackbot_app.parse_date("7-3-26").strftime("%Y-%m-%d") == "2026-07-03"
 
 
+@freeze_time(FROZEN)
+def test_four_digit_year_dash(slackbot_app):
+    assert slackbot_app.parse_date("7-3-2026").strftime("%Y-%m-%d") == "2026-07-03"
+
+
+@freeze_time(FROZEN)
+def test_two_digit_year_century_boundary(slackbot_app):
+    # Python's %y maps 00–68 → 2000s and 69–99 → 1900s. Pin it so nobody
+    # "fixes" the ambiguity by accident later.
+    assert slackbot_app.parse_date("12/31/99").year == 1999
+    assert slackbot_app.parse_date("1/1/68").year == 2068
+
+
 @freeze_time(FROZEN)
 def test_two_digit_year_past_not_rolled(slackbot_app):
     # Explicit two-digit year in the past stays put — no roll-forward.