feat: add nansen skills — 29 on-chain analytics skills (cherry-pick of #32)

[kevarifin14]

Cherry-pick of #32 (by @tonyagents) onto current main, with marketplace.json conflict resolved (merged with #31's nansen-dca-tracker entry).

Closes #32

Why this PR

#32 was A+ but had merge conflicts with .claude-plugin/marketplace.json after #11, #31, and earlier merges. Carries the same 29 skills with the conflict resolved so it can merge cleanly.

Review reference

Full A+ review at #32. Verification summary:

• nansen-cli v1.27.0 on npm, nansen-ai/nansen-cli repo exists (MIT)
• All 29 skill CLI commands cross-referenced against repo README
• All naming follows nansen-{name}
• Separate nansen-skills plugin block (now includes the pre-existing nansen-dca-tracker)
• No code; pure instructional
• Prerequisites (NANSEN_API_KEY + install) documented

Test plan

• CI checks pass
• 30 total nansen skills resolve from marketplace.json

Credit: @tonyagents (original author of #32)