Publish `bakery` as an installable Claude Code plugin

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.

Start by comparing the existing .claude/skills/bakery/ layout with the platform-skills pr-issue-linker plugin, then inspect bakery-skill-eval.yml and the sibling repository documentation. Move the skill, add the marketplace and plugin metadata, update triggers and install instructions, and verify the symlink and plugin installation without duplicating the skill in platform-skills.