New @swamp extension: extension quality rubric checker for CI

Problem

There is currently no way to automatically check an extension's quality rubric score as part of a CI pipeline. The swamp-extension-quality Claude skill can guide authors interactively through the 11-factor rubric, but this requires a human in the loop — it cannot be enforced programmatically on every pull request or push.

Proposed Solution

Publish a new @swamp/extension-quality extension (or add a method to an existing quality-checking extension) that:

• Accepts a local extension path or a published extension identifier as input
• Evaluates the extension against the 11-factor quality rubric (README, LICENSE, JSDoc coverage, etc.)
• Returns a structured score (percentage + letter grade) and per-factor pass/fail breakdown
• Exits non-zero (or sets a failing status output) when the score falls below a configurable threshold

This would allow CI workflows to include a step like:

- name: Check extension quality
  run: swamp model method run extension-quality check --input path=./extensions/models/my-ext

and gate merges on a minimum quality grade (e.g., B or above).

Alternatives Considered

• Running the swamp-extension-quality Claude skill in CI — requires an LLM call per run, is slow and expensive, and produces unstructured output that is hard to assert on.
• Writing a custom shell script — duplicates logic, diverges from the canonical rubric over time.

Additional Context

The 11-factor rubric is already well-defined and used by the swamp-extension-quality skill. A native extension would make it a first-class, versionable, CI-friendly check.