chore: remove openclaw plugin packages, use docker-only self-hosting

Delete both first-party OpenClaw plugin packages and the toolchain
surface around them:

- packages/openclaw-plugins/manifest (deprecated self-hosted plugin)
- packages/openclaw-plugins/manifest-model-router (cloud provider plugin)

Self-hosted users now run the Docker image directly; cloud users and
local agents configure Manifest as a generic OpenAI-compatible provider
in their OpenClaw config instead of installing a plugin. This removes
an entire maintenance surface that had become a source of flaky
installs, stale configs, and CI noise.

Changes:

- Drop both plugin packages from the npm workspace and turbo pipeline
- Remove publish:plugin / build:plugin / sync-plugin-version scripts
- Drop provider-plugin and manifest-plugin jobs from ci.yml
- Simplify changeset-check (backend/frontend no longer require a
  manifest changeset)
- Remove plugin publish, ClawHub publish, and version-triggered Docker
  build steps from release.yml (Docker is still built on PRs and via
  manual workflow_dispatch)
- Drop plugin package.json COPY steps from the Dockerfile and clean up
  .dockerignore plugin globs
- Delete the legacy three-step SetupWizard (SetupStepInstall /
  SetupStepConfigure / SetupStepVerify / SetupStepLocalConfigure) that
  was only referenced by its own tests. The active setup flow
  (SetupModal -> SetupStepAddProvider -> OpenClawSetup) already drives
  users through a plain OpenAI-compatible configuration and needs no
  plugin to work.
- Rewire CopyButton imports from SetupStepInstall to the standalone
  CopyButton component
- Remove pluginEndpoint from getAgentKey API type, backend controller,
  and all frontend consumers / tests
- Drop PLUGIN_OTLP_ENDPOINT env var and LOCAL_DEFAULT_PORT constant
  (both only existed to support the plugin's embedded server)
- Rewrite README, CLAUDE.md, and CONTRIBUTING.md to describe a
  Docker-only self-host path and remove every plugin mention
- Delete the four plugin-related developer skills under
  .claude/skills/ and the root skills/manifest ClawHub metadata

Author: brunobuddy