fix: use word-boundary lookaround for trigger phrase regex

[bledden]

Summary

• Replaces the (^|\s)...([\s.,!?;:]|$) trigger phrase regex with (?<!\w)...(?!\w) negative lookaround
• Patterns like (@claude), "@claude", >@claude, and cc:@claude now correctly trigger instead of being silently rejected
• Keeps existing protections against false positives inside words (e.g. email@claude.com still rejected)

Problem

The leading boundary (^|\s) only matched start-of-string or whitespace. Common patterns where the trigger phrase follows punctuation like (, ", >, or : were silently rejected — the GitHub workflow ran (because contains() is looser) but the action logged "No trigger was met" and exited with code 0.

The trailing boundary ([\s.,!?;:]|$) had the same issue — (@claude) failed on both sides since neither ( nor ) were in the allowed character sets.

Fix

Replaced both boundary groups with negative lookaround for word characters (\w = [a-zA-Z0-9_]):

// Before:
`(^|\\s)${escapeRegExp(triggerPhrase)}([\\s.,!?;:]|$)`

// After:
`(?<!\\w)${escapeRegExp(triggerPhrase)}(?!\\w)`

This matches the trigger phrase as long as it isn't embedded inside a word. Any non-word character (or start/end of string) is a valid boundary.

Test plan

• Added 8 new test cases covering all patterns from trigger_phrase regex rejects valid mentions preceded by non-whitespace chars like ( or " #941 ((@claude), "@claude", >@claude, cc:@claude, etc.)
• Added negative test for underscore boundary (test_@claude_test → rejected)
• All 653 existing tests pass
• TypeScript typecheck passes (bun run typecheck)
• Prettier format check passes (bun run format:check)

Fixes #941