Publish bakery as an installable Claude Code plugin #707

bakery's Claude Code skill currently lives only at .claude/skills/bakery/ in this repo, so it only auto-loads for people working directly in images-shared — it doesn't reach images-connect, images-workbench, images-package-manager, or images-examples, even though its own SKILL.md names all of them as its intended audience. Publishing it as a proper installable plugin fixes that: sibling repos could /plugin install bakery@images-shared directly, following the same pattern platform-skills already uses for its pr-issue-linker plugin.

Scope:

• Add a root .claude-plugin/marketplace.json and move the skill into plugins/bakery/{.claude-plugin/plugin.json,skills/bakery/{SKILL.md,evals/}}.
• Symlink .claude/skills/bakery to the new location so nothing breaks for the two current contributors; remove the symlink once both confirm the plugin install works.
• Update bakery-skill-eval.yml's path triggers and working-directory to match.
• Update sibling repos' README/CLAUDE.md with install instructions.

Separately: we'd like bakery discoverable from platform-skills too, without hand-maintaining a duplicate copy there — a second copy would drift the moment either one changes. claude-plugins-official's marketplace already has a git-subdir source type built for exactly this — pointing at a subdirectory of a different repo, pinned to a ref/sha, no vendoring:

"source": {
  "source": "git-subdir",
  "url": "https://github.com/posit-dev/images-shared.git",
  "path": "plugins/bakery",
  "ref": "main"
}

platform-skills doesn't have anything like a validate-skills.sh CI check yet (it has one eval-runner workflow so far, nothing that validates marketplace/plugin structure), so unlike the original posit-dev/skills proposal, there's no existing validation gap to flag here.