feat(python/agents): add crypto-payroll-agent sample

[plagtech]

Summary

Adds crypto-payroll-agent, a sample that batch-pays multiple
recipients in ETH or ERC-20 tokens on Base in a single transaction. The
agent uses the Spraay batch payment tools from
google-adk-community.

Pattern demonstrated

• Tool selection across four related tools: the agent picks among
  spraay_batch_eth, spraay_batch_token, and their _variable
  siblings based on whether the user wants equal or variable amounts
  and ETH or ERC-20.
• Tool composition: a local split_pool_proportionally helper feeds
  the variable batch tools when the user supplies a total and weights
  rather than per-recipient amounts.
• A local lookup_token_info helper resolves common Base tokens
  (USDC, WETH, cbBTC, DAI, etc.) so users don't need to know decimals.
• Human-in-the-loop confirmation with a configurable USD ceiling.

What's included

• crypto_payroll_agent/ — single LlmAgent with the four Spraay
  tools plus two local helpers
• eval/ — five-case evalset covering each batch-tool selection path
  and a safety refusal
• tests/ — 14 offline unit tests
• deployment/deploy.py — reference Vertex AI Agent Engine deployment
• README with four worked example interactions

Verification

• uv sync clean
• uv run pytest tests/ -v — 14 passed
• All four Spraay tools import successfully
• Agent loads in adk web

Note on the dependency

The Spraay tools are merged into google/adk-python-community main
but have not yet shipped in a PyPI release. The pyproject.toml pins
google-adk-community to git main for now; this can be relaxed to a
standard version constraint once a release ships.

Apache 2.0 license matches the repo default.