/skills dialog should sort skills alphabetically

The /skills dialog currently displays skills in an undefined order that is not alphabetical, not grouped by namespace or prefix, and not ordered by install time. This makes it difficult to find specific skills when many are installed.

Investigation findings:

• macOS APFS stores directory entries in a B-tree sorted by filename, so filesystem insertion order is not the cause
• The ordering does not match alphabetical sort by directory name, by the name: field in SKILL.md, or by namespace prefix
• The sort key appears to be some internal opaque value (possibly a hash or internal registration ID)
• Skills from the same namespace (e.g. gsd:*, superpowers:*, nd:*) are not grouped together

Example of observed disorder (local skills):
Directory names nd-security-assessment, nd-security-assessor, nd-tester, nd-testing appear in the dialog as: nd-security-assessor, nd-testing, nd-security-assessment, nd-tester — neither alphabetical by directory name nor by name: field and interspersed with the gsd:* skills.

Expected behavior: Skills should be grouped by namespace, then sorted alphabetically within each group. Grouping by namespace alone would already be a significant improvement.

Current behavior: Skills appear in an arbitrary order with no discernible grouping or sorting by name, namespace, prefix, or install time.

Environment:

• Claude Code v2.1.71
• macOS (darwin 25.3.0)
• Mix of local skills (~/.claude/skills/) and marketplace plugins