fix(skills): handle standalone GitHub skill URLs

[christso]

Summary

• fix standalone allagents skill add <GitHub skill URL> so the direct-install path uses the fetched source path only once
• keep the standalone GitHub URL flow on the direct-plugin path instead of falsely reporting Available skills: none
• add regression coverage for both URL path resolution and the standalone install flow

Root cause

installSkillFromSource() resolved the GitHub subpath before handing off to installSkillDirect(), and installSkillDirect() resolved the same subpath again. For URLs like https://github.com/NousResearch/hermes-agent/tree/main/skills/research/llm-wiki, that produced a non-existent path under the cache and made skill discovery return no skills.

Manual E2E

Red

cd /tmp/allagents-standalone-red-RBtPTs
node /home/entity/projects/EntityProcess/allagents.worktrees/fix-standalone-skill-source/dist/index.js skill add https://github.com/NousResearch/hermes-agent/tree/main/skills/research/llm-wiki
# before fix: Error: Skill 'llm-wiki' not found ... Available skills: none

Green

cd /tmp/allagents-standalone-red-RBtPTs
node /home/entity/projects/EntityProcess/allagents.worktrees/fix-standalone-skill-source/dist/index.js skill add https://github.com/NousResearch/hermes-agent/tree/main/skills/research/llm-wiki
# after fix: ✓ Enabled skill: llm-wiki (llm-wiki)
find .agents/skills -maxdepth 1 -mindepth 1
# shows .agents/skills/llm-wiki

Validation

bun test tests/unit/cli/skills-add-url-detection.test.ts tests/unit/cli/skills-add-standalone-install.test.ts
bun run lint
bun test
bun run build

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