fix(utils): parse multi-hyphen prefixes correctly (GH#405)

The ExtractIssuePrefix function was falling back to first-hyphen
extraction when the suffix didn't look like a hash (e.g., 4+ char
words without digits). This broke prefixes like 'hacker-news' where
an issue ID 'hacker-news-test' would incorrectly extract 'hacker'.

Fix: Always use last-hyphen extraction for alphanumeric suffixes.
Only fall back to first-hyphen for non-alphanumeric suffixes.

Examples:
- 'hacker-news-test' -> 'hacker-news' (was: 'hacker')
- 'me-py-toolkit-abc' -> 'me-py-toolkit' (was: 'me')
- 'vc-baseline-hello' -> 'vc-baseline' (was: 'vc')

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.beads/.gitignore

@@ -30,3 +30,5 @@ beads.right.meta.json
 !issues.jsonl
 !metadata.json
 !config.json
+deletions.jsonl
+deletions.jsonl.migrated