#22221: fix: preserve case of target ID in normalizeTargetId

## Summary Fixes #22217 Slack user IDs are case-sensitive (e.g. `U3UR2BMQ8`). `normalizeTargetId()` was lowercasing the entire `kind:id` string, converting `channel:U3UR2BMQ8` to `channel:u3ur2bmq8`, causing Slack API `channel_not_found` errors when opening DM channels. ## Changes - Only lowercase the `kind` prefix, preserve the original case of `id` ## Before / After | | Result | |---|---| | Before | `channel:u3ur2bmq8` ❌ | | After | `channel:U3UR2BMQ8` ✅ | --- 🤖 Generated with Claude Code <!-- greptile_comment --> <h3>Greptile Summary</h3> Fixed Slack API `channel_not_found` errors by preserving case-sensitivity in user/channel IDs. The `normalizeTargetId()` function now only lowercases the `kind` prefix (`user`/`channel`) while maintaining the original case of the ID portion (e.g., `U3UR2BMQ8`). - Changed normalization from `"channel:u3ur2bmq8"` to `"channel:U3UR2BMQ8"` - Fixes integration with Slack API which requires case-sensitive IDs - Tests in `src/slack/targets.test.ts` need updating to reflect new normalized format <h3>Confidence Score: 4/5</h3> - Safe to merge after updating test expectations - The fix correctly addresses the Slack API case-sensitivity issue with a minimal, focused change. However, the existing tests expect lowercase normalized IDs and will fail without updates. Once tests are updated to reflect the new behavior, this should be safe to merge. - Check `src/slack/targets.test.ts` - test expectations need updating _{Last reviewed commit: ddd78e6} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->