docs: add canonical tangent agent skills

[Volv-G]

Summary

• Canonicalizes the Tangent agent skill bundle under skills/tangent/ in tangle-cli.
• Updates stale lab/pre-publication/package guidance for the current public package layout.
• Uses uv-first install/invocation guidance: uv run from a checkout, uv tool install tangle-cli for a persistent CLI, and uvx --from tangle-cli tangle ... for one-off execution.
• Includes skills/** in the source distribution via packaging config.
• Adds packaging tests for skill bundle presence, stale wording guards, uv-first guidance, and sdist inclusion.

Tests

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

Notes

• No package release/publish in this change.
• Lab-side follow-up is not required by default because lab main has no skills/ tree; the lab branch remains provenance for the original bundle.