fix(skills): move parallel-cli to optional-skills (#3673)

parallel-cli is a paid third-party vendor skill that requires
PARALLEL_API_KEY, but it was shipped in the default skills/ directory
with no env-var gate. This caused it to appear in every user's system
prompt even when they have no Parallel account or API key.

Move it to optional-skills/ so it is only visible through the Skills
Hub and must be explicitly installed. Also remove it from the default
skills catalog docs.

website/docs/reference/skills-catalog.md

@@ -254,7 +254,6 @@ Skills for academic research, paper discovery, literature review, domain reconna
 | `blogwatcher` | Monitor blogs and RSS/Atom feeds for updates using the blogwatcher CLI. Add blogs, scan for new articles, and track what you've read. | `research/blogwatcher` |
 | `domain-intel` | Passive domain reconnaissance using Python stdlib. Subdomain discovery, SSL certificate inspection, WHOIS lookups, DNS records, domain availability checks, and bulk multi-domain analysis. No API keys required. | `research/domain-intel` |
 | `duckduckgo-search` | Free web search via DuckDuckGo — text, news, images, videos. No API key needed. Use the Python DDGS library or CLI to search, then web_extract for full content. | `research/duckduckgo-search` |
-| `parallel-cli` | Optional vendor skill for Parallel CLI — agent-native web search, extraction, deep research, enrichment, FindAll, and monitoring. | `research/parallel-cli` |
 | `ml-paper-writing` | Write publication-ready ML/AI papers for NeurIPS, ICML, ICLR, ACL, AAAI, COLM. Use when drafting papers from research repos, structuring arguments, verifying citations, or preparing camera-ready submissions. Includes LaTeX templates, reviewer guidelines, and citation verificatio… | `research/ml-paper-writing` |
 | `polymarket` | Query Polymarket prediction market data — search markets, get prices, orderbooks, and price history. Read-only via public REST APIs, no API key needed. | `research/polymarket` |