docs(agents): add a page for giving an agent a wallet and test funds

[GigaHierz]

Part of #2263 (first of up to three PRs; the x402 clarifications and jargon pass follow separately).

The agent pages read AGENT_PRIVATE_KEY from the environment, but no page said where the key comes from or how to fund it on testnet. This adds that page.

Changes

• New build/agents/agent-wallet.mdx: generate an account with viem, get CELO from the Celo Sepolia faucet and USDC from the Circle faucet, send a first transfer, with a short troubleshooting table.
• Added to the Agents nav group after use-docs-with-ai.
• Linked from the "Giving an agent a wallet" section of build/agents/overview and from the Celo Sepolia bullet on the x402 page.

Verification

• npx mint broken-links: no broken links found.
• python3 scripts/check_redirects.py: no redirect problems found.
• Faucet behaviour (CELO only, 1 CELO per unauthenticated request, USDC from the Circle faucet) was read from the faucet page itself.
• Not run: the two scripts were not executed. I had no funded testnet key in this environment, and the celoSepolia chain export is taken from how the repo already imports it from wagmi/chains. Please run generate-key.mjs and first-transfer.mjs once before merging; AGENTS.md section 5 requires it.

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