fix: point skill prompt at exact custom skill paths

[LittleChenLiya]

Summary

• expose each injected skill's exact absolute SKILL.md path as a path attribute in the prompt
• explicitly tell the model not to guess alternate skill directories and clarify the /mnt/skills/public vs /mnt/skills/custom split
• add prompt coverage for custom skill paths so nested custom skills keep their exact runtime location

Root cause

The skills prompt told the model to use a path attribute from each skill tag, but the tag only exposed a nested <location> field and did not spell out the custom-skill directory convention clearly. That left the model to infer custom skill paths on its own, which could send it to the wrong directory.

Fixes #2083.

Testing

• uv run --directory backend pytest tests/test_lead_agent_skills.py -q
• uv run --directory backend python - to inspect the generated prompt and verify a custom skill is injected as /mnt/skills/custom/.../SKILL.md