Feature request: support external repo references in marketplace.json

## Summary Currently, \`anthropics/skills\` is a copy-in monorepo — every community skill is committed as a full copy with no link back to the contributor's source repo. This creates a maintenance problem: once a PR is merged, the copy in this repo diverges from the contributor's source as they continue iterating. ## The problem - Community skills are snapshots frozen at PR submission time - Contributors who actively maintain their skills (adding features, fixing issues, improving documentation) have no mechanism to propagate updates - Submitting update PRs manually doesn't scale — a contributor with 5+ skills who ships improvements weekly would need to file constant sync PRs - Users who discover a skill here have no reliable way to get the latest version ## Proposed solution Add support for external repo references in \`marketplace.json\`, so a skill entry can point to an external repo rather than (or in addition to) a local copy: \`\`\`json { "name": "synthesis-codebase-review", "source": "external", "repo": "https://github.com/rajivpant/synthesis-skills", "path": "synthesis-codebase-review/SKILL.md", "version": "1.0.0" } \`\`\` With this, the Anthropic directory becomes a discovery channel. \`npx skills add\` resolves to the contributor's repo, which always has the latest version. ## Why this matters Anthropic already has an internal precedent: the \`claude-api\` skill uses \`auto-sync/\` branches (see PRs #729-730) to automate updates. External repo references would extend a similar capability to community contributors. ## Alternative: contributor-maintained sync branches If external references are architecturally complex, a lighter alternative would be a documented pattern for contributors to submit PRs from a dedicated sync branch in their repo, with a webhook or CI action that opens update PRs when the source changes. ## Context I submitted PR #690 with 5 skills from [github.com/rajivpant/synthesis-skills](https://github.com/rajivpant/synthesis-skills). Within days of submission, all 5 skills received substantive updates — new frontmatter metadata, content revisions, version bumps — that the PR snapshot cannot reflect. This feature request is grounded in that direct experience. The full collection (20 skills, CC0-1.0) is actively maintained and would benefit from a sync mechanism rather than periodic manual PRs.