Add Herd plugin for onchain research, simulation, and bookmarks

[andrewhong5297]

Summary

Adds skills/base-mcp/plugins/herd.md — a new external-mcp plugin that connects Base MCP users to Herd, an onchain research and simulation platform for Base and Ethereum.

What Herd adds alongside Base MCP

• Research & Exploration — wallet overviews (type detection, balances, tx count), contract metadata, AI-powered source code search, transaction history, event/function filtering, and token activity across Base and Ethereum.
• Bookmarks (Onchain Memory) — persistent labels for wallets, contracts, and transactions that survive across sessions. The agent bootstraps each session from bookmarks, so users don't repeat addresses. Bookmarks link to human-readable herd.eco explorer pages.
• HAL Simulation — simulate arbitrary transactions in a sandboxed Tevm fork via Herd MCP, inspect the full oplog, then pipe verified unsigned calls to Base MCP's send_calls before the user approves anything.

Plugin spec conformance

Field	Value
integration	external-mcp
chains	[base, ethereum]
tags	[research, explorer, simulation, bookmarks]
auth	oauth-on-install (API key also supported, beta)
risk	[irreversible]
shell	none

Sections present in canonical order: [!IMPORTANT] callout, ## Overview, ## Detection, ## Installation, ## Surface Routing, ## Orchestration, ## Submission, ## Example Prompts, ## Risks & Warnings, ## Notes.

Per Contribution Scope, this PR only touches skills/base-mcp/plugins/herd.md. SKILL.md and the conformance table are left for maintainers.

Herd MCP

• MCP URL: https://mcp.herd.eco/v1
• Explorer: https://herd.eco
• Docs: https://docs.herd.eco

[cb-heimdall]

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