[Plugins] Tutorial and reference docs for plugin authors and consumers #1075

Part of #1068.

Scope

Author an end-to-end tutorial and reference documentation for the plugin system.

Requirements

• Tutorial walks a new author through: scaffold a plugin -> write a manifest -> validate -> dry-run -> install into a sample Squad project -> enable -> switch -> uninstall.
• Tutorial completes in under 30 minutes for someone already familiar with Squad.
• Reference docs cover:
  • plugin.manifest.json schema (every field, required/optional, examples).
  • Install request format.
  • plugin.lock.json format.
  • Full squad plugin CLI surface.
  • Security posture: what the installer will and will not do.
  • Versioning policy (SemVer + prerelease channels).
• All commands in the tutorial are exercised by a CI smoke test so the docs cannot silently rot.

Acceptance

• Tutorial published under the docs site / docs directory.
• Reference docs published and linked from the epic.
• CI smoke test runs the tutorial commands end-to-end.

Planning provenance: tamirdresher_microsoft/tamresearch1#2926.