Add llm-prompt-lint to plugin directory

[HadiFrt20]

Adds llm-prompt-lint to the Extra commands section of the plugin directory.

What it does:

• llm prompt-lint <file> — static analysis with 8 rules (conflicting constraints, role confusion, vague language, missing examples, no injection guard, etc.)
• llm prompt-score <file> — quality score 0-100 across 5 dimensions (structure, specificity, examples, safety, completeness)
• llm prompt-diff <a> <b> — semantic diff that matches sections by type, classifies changes, and rates impact

All commands support --json for CI pipelines.

Works with .prompt files (YAML frontmatter + labeled sections) and plain text system prompts.

Example:

llm install llm-prompt-lint
llm prompt-lint my-agent.prompt
llm prompt-score my-agent.prompt
llm prompt-diff v1.prompt v2.prompt

GitHub: https://github.com/HadiFrt20/llm-prompt-lint