fix(add): discover explicitly named nested skills

[EfeDurmaz16]

Summary

• search full depth when skills add has an explicit --skill filter
• skip the GitHub blob fast path for those named installs so nested skills are available from the cloned tree
• add a regression test for an AWS-style deeply nested aws-transform skill alongside a shallow skill

Fixes #1148.

Verification

• pnpm install
• pnpm format
• pnpm format:check
• pnpm exec vitest run src/add.test.ts -t "should discover deeper nested skills" --testTimeout=15000
• pnpm exec vitest run src/add.test.ts --testTimeout=15000
• pnpm build

Blocked / known failing checks

• pnpm type-check currently fails on unrelated upstream type errors in src/git.ts and src/skills.ts.