fix: make trigger_phrase match case-insensitive (#1279)

GitHub @-mention autocomplete inserts @Claude (capitalized) when users
pick the bot from the dropdown, but the trigger regex had no 'i' flag,
so the action would log 'No trigger was met for @claude' and exit. The
workflow's outer 'if: contains(...)' gate is case-insensitive, so the
job runs and looks like it silently ignored the user.

The regex was case-sensitive since the initial commit with no test
asserting either behavior; the existing tests focus on word-boundary
precision (email@claude.com etc.), not case.

test/trigger-validation.test.ts

@@ -186,6 +186,8 @@ describe("checkContainsTrigger", () => {
         { issueBody: "@claude: here's the issue", expected: true },
         { issueBody: "@claude; and another thing", expected: true },
         { issueBody: "Hey @claude, can you help?", expected: true },
+        { issueBody: "@Claude can you help?", expected: true },
+        { issueBody: "@CLAUDE fix this", expected: true },
         { issueBody: "claudette contains claude", expected: false },
         { issueBody: "email@claude.com", expected: false },
       ];