refactor: use strings.HasPrefix in agentkit test prefix matching

[gxmiranda]

Summary

Replace fragile manual len()+slice prefix matching with strings.HasPrefix in TestScaffold_FileCount.

Problem

The test in internal/agentkit/agentkit_test.go uses manual length-check plus raw slice indexing to categorize scaffold results by directory prefix:

case len(r.Path) > 9 && r.Path[:9] == "commands/":

The integer constants (9, 7, 7) must be manually kept in sync with the prefix string lengths. An off-by-one error is easy to introduce, as happened during the command/ → commands/ rename in PR #20.

Solution

Use strings.HasPrefix which is immune to length/index mismatches:

case strings.HasPrefix(r.Path, "commands/"):

Changes

• Replace 3 manual len()+slice prefix checks with strings.HasPrefix
• Add "strings" import
• Include OpenSpec change artifacts

Closes #22

[yvonnedevlinrh]

This is a well-scoped, low-risk refactor with proper spec artifacts, passing CI, and full alignment with the constitution. The change eliminates a real maintenance hazard (manual index constants) with a proven stdlib alternative.