Improve workspace repo navigation and TOC defaults

[gas2own]

What

• add a GitHub sidebar tool to the workspace Quarto scaffold and open it in a new tab
• teach the workspace skill to fill the canonical repo URL
• place the table of contents on the left so it does not obscure wide content

This upstreams the defaults validated in allenai/meta-agent-notes#2.

Tracked by allenai/gas2own#388.

Validation

• ruff format --check .
• ruff check .
• skill frontmatter validation (all 24 valid)
• rendered-template YAML assertion for repo URL, _blank, and left TOC
• pytest: 189 tests pass locally; the one npx-install test cannot run because the dispatcher npm cache is root-owned (GitHub CI will run it in a clean environment)