ci: rename plugin branch to plugins and document plugin install

[xukai92]

What

• Renames the workflow that publishes the bundled Claude Code plugin from plugin -> plugins (matching the /plugin marketplace add ... UX).
• Renames .github/workflows/plugin.yml -> .github/workflows/plugins.yml and updates the workflow display name accordingly.
• Adds a new Install as a Claude Code Plugin section to the README explaining how to install re:factory via the marketplace from the plugins branch, and clarifies how that path differs from factory install.

Why

The branch and workflow name now match what a user actually types: /plugin marketplace add akashgit/remote-factory#plugins. Previously the README had no instructions for using the bundled plugin at all, even though the workflow was already publishing it.

Follow-up

The existing plugin branch on origin is untouched by this PR; delete it manually after merge.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.38%. Comparing base (4370276) to head (9fdcc84).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #397   +/-   ##
=======================================
  Coverage   87.38%   87.38%           
=======================================
  Files          61       61           
  Lines        9396     9396           
=======================================
  Hits         8211     8211           
  Misses       1185     1185           

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[colehurwitz]

Review

Docs + CI rename. Verified the README claims against the repo (manifests, factory install help, skills/ contents) — everything lines up.

Overview

• Renames .github/workflows/plugin.yml → plugins.yml, the workflow display name, and the published branch (plugin → plugins) so they match the /plugin marketplace add akashgit/remote-factory#plugins UX.
• Adds an Install as a Claude Code Plugin section to the README and reframes the existing Plugin Agents section as the "skip the marketplace" alternative, with a new --runner codex example.

Correctness

• Workflow trigger (push: branches: [main]) matches the README's "rebuilds the plugins branch on every push to main". ✅
• /plugin install factory@remote-factory matches .claude-plugin/marketplace.json (remote-factory marketplace, factory plugin). ✅
• /factory:implement is the only slash command shipped (commands/implement.md). ✅
• pipeline-subagents and implement skills both exist under skills/. ✅
• factory install --runner codex → ~/.codex/agents/ matches factory/cli.py:3724. ✅
• Grep confirms no other docs/workflows reference the old plugin branch name, so no orphaned references after the rename.

Nits (non-blocking)

• plugins.yml still has jobs.build-plugin: (singular) and the commit message string "build: generate plugin agent files from …". Cosmetic only — "plugin" reads fine as an adjective there — but if you want full consistency, jobs.build-plugins would match the rename.
• First post-merge gap: until the next push to main actually fires plugins.yml, the new plugins branch won't exist on origin. Worth merging a trivial follow-up commit (or running the workflow manually) right after merge so the README instructions don't 404 for early adopters.
• The "Follow-up: delete the existing plugin branch manually" is fine, but anyone who already ran /plugin marketplace add …#plugin will silently get a frozen snapshot once main advances past the deletion. A one-liner in the new README section telling existing users to re-add with #plugins would be a nice touch — not required for this PR.

Risk

Minimal. No runtime code paths touched; the only behavioral change is which remote branch the workflow force-pushes to.

Verdict: LGTM — ship it, optionally fold in the jobs.build-plugins rename before merge.