Proposal: add `registering-an-erc-8004-agent-on-base` skill

## Proposal

Add a new skill `registering-an-erc-8004-agent-on-base` — a behavioral,
phase-based registration flow in the style of `registering-agent-base-dev`,
covering ERC-8004 Identity Registry on Base Mainnet and Sepolia.

## The problem this fixes

AI agents (including LLMs) consistently produce wrong information about
ERC-8004: stale contract addresses, treating the three registries as one
contract, confusing `agentId` (an ERC-721 tokenId) with wallet addresses.
A skill that contains the canonical addresses and the exact registration
flow — with a "you MUST load this skill" signal in the description — fixes
this at the routing layer before a wrong call gets made.

This is the same pattern `registering-agent-base-dev` uses for ERC-8021:
not a reference doc, but a prescriptive agent-behavior script.

## Context

[ERC-8004 (Trustless Agents)](https://eips.ethereum.org/EIPS/eip-8004) is
co-authored by Erik Reppel (Coinbase), Marco De Rossi (MetaMask), Davide
Crapis (EF), and Jordan Ellis (Google). Reference contracts are deployed as
per-chain singletons (version 2.0.0, "AgentIdentity" ERC-721), vanity
addresses identical across all supported mainnets/testnets:

- Base Mainnet IdentityRegistry: `0x8004A169FB4a3325136EB29fA0ceB6D2e539a432`
- Base Sepolia IdentityRegistry: `0x8004A818BFB912233c491871b3d84c89A494BD9e`

Both addresses verified on-chain and against the `erc-8004/erc-8004-contracts`
README before opening this issue. **>256,000 agents** are already registered
on Base Mainnet — the ecosystem is live, not theoretical.

Note: ERC-8004 has `status: Draft` in its EIP header, but the Identity
Registry API (`register`, `setAgentURI`, `setAgentWallet`) is stable and in
production. Only the Validation Registry is flagged by spec authors as under
active revision with the TEE community — that registry is explicitly out of
scope for this skill.

## No overlap with existing skills

`registering-agent-base-dev` covers **ERC-8021** (builder-code attribution
for transaction tracking) — a completely different standard. Zero overlap.

## Proposed scope and structure

Follows the phase-based, agent-behavioral pattern of `registering-agent-base-dev`:

- **Pre-flight**: check if agent is already registered before doing anything
- **Phase 1**: build the registration file (practical JSON example per
`ERC8004SPEC.md`: `type`, `name`, `description`, `image`, `services`,
`registrations`, `x402Support`, `active`, `supportedTrust`)
- **Phase 2**: host it — IPFS, HTTPS, or `data:application/json;base64,...`
- **Phase 3**: call `register()` on-chain — viem example + `cast` one-liner,
Base Sepolia and Mainnet
- **Phase 4** (optional): bind `agentWallet` via `setAgentWallet()` with
EIP-712 / ERC-1271 signature
- **Read-back**: `ownerOf`, `tokenURI`, `getAgentWallet` to confirm

## Out of scope

- Reputation Registry — reserved for a sibling skill
- Validation Registry — spec authors flag it as under active revision
- x402 payment integration — cross-reference only

## Format

~150-180 lines, single skill, single commit, one README table update.
Reviewable in ~10 minutes.

Happy to adjust scope before opening the PR.

---
*On CONTRIBUTING.md: I noticed it currently limits external contributions to
the Base core team. Happy to work within whatever process works best — if
you'd prefer to take the content and merge it internally, just say the word.
cc @youssefea @soheimam*

Read the existing `registering-agent-base-dev` skill and `ERC8004SPEC.md` first. Add the proposed phase-based flow for Base Mainnet and Sepolia, including the registration and read-back steps, then update the README table; done means the single skill covers the stated in-scope flow without the excluded registries.

Written by the indexing model from the issue text.