fix(vendor-neutrality-score): never count partial-coverage tools as backends

[potiuk]

Summary

• tools/vendor-neutrality-score counted every **Kind:** implementation tool as a backend vendor and never read the **Coverage:** qualifier. docs/labels-and-capabilities.md says a partial tool "must not be counted as a complete/selectable backend", but nothing enforced it. A read-only foundation for a new forge (for example the GitLab bridge in feat(tools/gitlab): add read-only GitLab bridge foundation (Part of #305) #1369) would raise the contract counts in docs/vendor-neutrality.md as if it were a full backend, even after its README declared **Coverage:** partial.
• The scorer now reads **Coverage:** (partial or partial-read-only, with or without backticks), keeps partial tools out of the vendor count, lists them in the generated Basis column (partial foundation, not counted: …) and in --json (partial_implementations), and fails on unknown values, just as it already does for **Kind:**.
• No score changes today. Atlassian stays counted for contract:tracker / contract:change-request through jira / jira-patch, and bitbucket now appears as not counted.
• The coverage-qualifiers section now also documents partial, which tools/bitbucket/README.md already uses, and states that the scorer enforces it.

Type of change

• Python package (tools/*/ with pyproject.toml)
• Documentation (docs/)

Test plan

• uv run --directory tools/vendor-neutrality-score pytest: 36 passed (3 new: partial not counted; README parsing incl. backticked value; unknown value raises)
• ruff check, ruff format --check, mypy for the tool: clean
• vendor-neutrality-score --in-place regenerated the doc block (Basis column only; counts unchanged)
• prek run --all-files: all hooks pass except workspace-pytest for dev, agent-isolation and pr-management-stats, which fail identically on unmodified main in my sandboxed environment (socket / subprocess denial, local plugin tree). CI runs them unsandboxed.

Linked issues

Follow-up from the review of #1369.

Generative AI disclosure

This change was drafted with Claude Code (Claude Opus 5) and reviewed by me before submission.

🤖 Generated with Claude Code