feat: list sbx-kits (Docker Sandboxes kit authoring skill)

[jedp-docker]

Summary

• Adds a sbx-kits entry to plugins[] referencing docker/sbx-kits-contrib by git source ({"source": "github", "repo": "docker/sbx-kits-contrib", "ref": "main"}) rather than vendoring the skill's files into this repo. This is the first external-source entry in this marketplace — existing entries all use the ./plugins/... local-path convention, but that's a convention, not a schema requirement.
• The skill is sbx-kits-contrib's kit-author skill: reference documentation for authoring Docker Sandboxes kits (spec.yaml, v2 schema).
• Entry is named sbx-kits, matching that repo's own .claude-plugin/plugin.json name (feat: add plugin manifest, fix kit-author skill activation trigger sbx-kits-contrib#126). Component namespacing (<plugin-name>:<skill-name>) is driven by plugin.json's own name, while the marketplace entry's name is only the install alias; a mismatch would mean /plugin install X@docker produces something invoked as Y:skill-name, which is confusing. sbx-kits is a deliberate, user-facing namespace choice, not tied to the source repo's name — chosen so that if more skills fold in later (e.g. workflow skills currently living in sbx-toolkit), they all land under this one sbx-kits: prefix. Namespacing has no cross-repo merging mechanism, so any future skills sharing this prefix must live in the same sbx-kits-contrib repo/plugin, not a separately-sourced one.

Depends on

• feat: add plugin manifest, fix kit-author skill activation trigger sbx-kits-contrib#126 — adds .claude-plugin/plugin.json and trims/fixes the skill's activation trigger, which is what makes that repo a valid, discoverable plugin target. This PR's ref: main will not resolve to a usable plugin until #126 merges. Opening as draft for that reason; will mark ready once #126 is in and, ideally, once a release tag exists to pin ref to instead of the mutable main branch.
• fix(kit-author): correct resources field name and validation rule sbx-kits-contrib#125 — unrelated doc fix, no dependency, listed for context.

Test plan

• marketplace.json is valid JSON.
• After #126 merges: /plugin marketplace add docker/claude-plugins → /plugin install sbx-kits@docker successfully discovers skills/kit-author/ from the referenced repo, invoked as /sbx-kits:kit-author.
• Follow-up: pin ref to a release tag/SHA instead of main once one is cut.