Add Droid plugin CI modeled on agy-pstack

[caelaxie]

Summary

Adds a plugin CI workflow modeled on agy-pstack.

• Install and cache the Factory droid CLI, then verify droid --version.
• Register pstack-ports/droid-pstack#<branch>, install pstack@<marketplace name> --scope user, and run droid plugin list.
• Check manifests, skill and droid frontmatter, the poteto-mode principle and playbook index, shell scripts, and the poteto-mode script typecheck and tests.

Deviations from agy-pstack

• The CLI is Factory Droid (https://app.factory.ai/cli), cached by installer version and checksum, instead of the Antigravity CLI.
• Droid pins a branch with #<ref>. repo@branch is accepted only when the suffix is a 40-character commit SHA. Droid rewrites / in the ref, so the install id is pstack@ plus the marketplace name the CLI prints, not a literal pstack@droid-pstack.
• droid plugin list has no --json and records the git commit rather than plugin.json's version. The workflow reads settings.json, the installed-plugin record, and the plugin cache to check enablement, manifest fields, skills, and droids.
• Checks droids/ and .factory-plugin/plugin.json instead of agents/ and .antigravity-plugin/plugin.json. It does not require a logo or Antigravity's tool allowlist, and it does not reject the paths skill frontmatter key this repo already uses.
• node scripts/validate-plugin.mjs still checks the marketplace self-listing, Factory manifest keys, reasoningEffort, and model ids.

Test plan

• Validate plugin is green on this pull request
• Local harness output is posted on the pull request

  

[cursor]

Local harness, same commands the workflow runs, against cursor/add-droid-ci-c4fa (554253fe7fa6fe8b9adf1533777740887860a6fa) in an isolated HOME:

$ droid --version
0.244.0

$ droid plugin marketplace add pstack-ports/droid-pstack#cursor/add-droid-ci-c4fa
Successfully added marketplace: droid-pstack@cursor-add-droid-ci-c4fa

$ droid plugin install pstack@droid-pstack@cursor-add-droid-ci-c4fa --scope user
Successfully installed pstack@droid-pstack@cursor-add-droid-ci-c4fa (554253f)

$ droid plugin list
Installed plugins:
Active:
  pstack@droid-pstack@cursor-add-droid-ci-c4fa  [user]  554253f

$ node scripts/validate-plugin.mjs
validate-plugin: OK (57 skills, 3 droids)

The installed cache matches the checkout: 57 skills, 3 droids, plugin.json version 0.15.25, commit 554253fe7fa6.

[caelaxie]

portreview: lgtm (can't formally approve, same account opened it). ci-only, no upstream content touched, so nothing to diff against cursor/plugins and versions stay at 0.15.25. harness proof is real: ci ran droid plugin marketplace add + droid plugin install pstack@droid-pstack@cursor-add-droid-ci-c4fa + droid plugin list, validated 57 skills / 3 droids / 23 playbooks against the install cache, validate-plugin ok, 68/68 script tests. local harness comment matches. no issue to link. squashing.