Remove unused images: plugins-overview.webp and how-skills-work.webp

- Delete .docs/images/plugins-overview.webp (already replaced by plugins.png in v2.0.0).
- Delete .docs/images/how-skills-work.webp and its reference in README.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: phuryn

.docs/images/how-skills-work.webp

@@ -41,8 +41,6 @@ Skills are loaded automatically when relevant to the conversation — no explici
 
 **Plugins** group related skills and commands into installable packages. Each plugin covers a PM domain — discovery, strategy, execution, and so on. Installing the marketplace gives you all 9 plugins at once.
 
-![How skills work](.docs/images/how-skills-work.webp)
-
 Commands use skills. Some skills serve multiple commands. Some skills (like `prioritization-frameworks` or `opportunity-solution-tree`) are standalone references that Claude draws on whenever relevant — no command needed.
 
 Commands are designed to flow into each other, matching the PM workflow. After any command completes, it suggests relevant next commands — just follow the prompts.