docs: clarify lazy skill source host paths

[alfozan]

Summary

• Clarify that LocalDirLazySkillSource(source=LocalDir(src=...)) reads from the SDK host filesystem.
• Distinguish LocalDir.src from the in-sandbox skills_path destination.
• Add matching comments to sandbox examples that configure LocalDirLazySkillSource.

Test plan

• make format
• make lint
• make build-docs
• uv run pytest -n auto --dist loadfile -m "not serial" --ignore=tests/local

Not fully green locally due unrelated environment/local-checkout issues:

• Full make tests collects ignored tests/local/old/... files in this checkout and fails importing customer_support_bot.
• Serial tests with tests/local ignored fail during Docker/Testcontainers setup because Colima cannot create the Docker socket mount path.
• make typecheck fails on existing ignored tests/local files plus existing Temporal example typing issues.

Issue number

Closes #2991

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass where not blocked by local environment issues

[seratch]

Thank you!