docs: prefer published tangle-cli in tangent skills

[Volv-G]

Summary

• Makes the published tangle-cli package the default Tangent skill usage path.
• Recommends uv tool install tangle-cli for persistent CLI usage and uvx --from tangle-cli tangle ... for one-off commands.
• Frames checkout uv run tangle ... as local tangle-cli development/validation only.
• Normalizes agent/reference command examples to bare tangle ... commands.
• Updates wording guards in packaging tests.

Tests

• uv run pytest tests/test_packaging.py -q
• uv lock --check
• uv run pytest -q
• git diff --check

Notes

• Follow-up to docs: add canonical tangent agent skills #14.
• No package release/publish in this change.