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.

src/github/validation/trigger.ts

@@ -51,6 +51,7 @@ export function checkContainsTrigger(context: ParsedGitHubContext): boolean {
     // Check for exact match with word boundaries or punctuation
     const regex = new RegExp(
       `(^|\\s)${escapeRegExp(triggerPhrase)}([\\s.,!?;:]|$)`,
+      "i",
     );
 
     // Check in body
@@ -77,6 +78,7 @@ export function checkContainsTrigger(context: ParsedGitHubContext): boolean {
     // Check for exact match with word boundaries or punctuation
     const regex = new RegExp(
       `(^|\\s)${escapeRegExp(triggerPhrase)}([\\s.,!?;:]|$)`,
+      "i",
     );
 
     // Check in body
@@ -105,6 +107,7 @@ export function checkContainsTrigger(context: ParsedGitHubContext): boolean {
     // Check for exact match with word boundaries or punctuation
     const regex = new RegExp(
       `(^|\\s)${escapeRegExp(triggerPhrase)}([\\s.,!?;:]|$)`,
+      "i",
     );
     if (regex.test(reviewBody)) {
       console.log(
@@ -125,6 +128,7 @@ export function checkContainsTrigger(context: ParsedGitHubContext): boolean {
     // Check for exact match with word boundaries or punctuation
     const regex = new RegExp(
       `(^|\\s)${escapeRegExp(triggerPhrase)}([\\s.,!?;:]|$)`,
+      "i",
     );
     if (regex.test(commentBody)) {
       console.log(`Comment contains exact trigger phrase '${triggerPhrase}'`);