Point catalog sync at skills/core/

[ericciarla]

Half 1 of renaming the catalog's skills/cli/ → skills/core/ (the family covers the Firecrawl primitives via CLI or MCP, not just the CLI — see firecrawl/skills for the other half).

• sync-catalog.yml now mirrors into catalog skills/core/ and writes a "Core skills (mirror)" README
• Updated the skills-install.ts doc comments; no behavior change — install selection is name-based, so skills.sh slugs and firecrawl init are unaffected

Merge this first. The sync will create skills/core/ in the catalog alongside the old folder; the catalog PR then removes skills/cli/ and updates the plugin manifests, so there's no window where manifest paths point at a missing directory.

🤖 Generated with Claude Code

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Summary by cubic

Rename the catalog mirror from skills/cli/ to skills/core/ to reflect that these are shared core skills used by CLI and MCP. No runtime behavior changes; install selection remains name-based, so slugs and firecrawl init are unchanged.

• Update .github/workflows/sync-catalog.yml to mirror cli/skills/ into catalog skills/core/ and write a "Core skills (mirror)" README.
• Update doc comments in src/commands/skills-install.ts to reference skills/core/ only.

Review and rollout

• Merge this first. The sync will create skills/core/ alongside skills/cli/.
• Follow-up in the catalog will remove skills/cli/ and update plugin manifests, avoiding any window with missing paths.
• No user or integrator actions required.

^{Written for commit 7a48d59. Summary will update on new commits.}