feat: add financial_governance spec block and purchasing-agent example

[Danbi58]

Demonstrates the proposed financial_governance block in a realistic
purchasing agent scenario with spending caps, category allowlists,
human approval threshold, and Slack/email/SMS notifications.

What

Adds two things:

1. examples/financial-agent/agent.yaml — a complete purchasing
   agent example demonstrating the proposed financial_governance
   block in a realistic compliance scenario
2. The financial_governance block itself — a new optional spec
   addition that lets payment-capable agents declare runtime
   financial controls directly in their agent definition

Why

The compliance block handles agent identity, SOD, and audit
logging at the definition level. But there's currently no standard
way to declare runtime financial controls — spending caps,
category allowlists, human approval thresholds, and which financial
firewall enforces them.

Recent incidents illustrate why this matters:

• Feb 2026: Compromised API key ran up $82,314 in 48 hours with
  no spending cap to limit the damage
• Nov 2025: LangChain agent loop ran 11 days undetected — $47,000
• Feb 2026: Decimal parsing error caused an agent to send $441,000
  to a random address with no transaction cap

The financial_governance block fills this gap. It is additive,
optional, disabled by default, and ignored cleanly by exporters
that don't implement it.

Closes #38

How Tested

• YAML is valid and parses correctly
• Example follows existing style in examples/ directory

Checklist

• My code follows the existing style of this project
• I have updated documentation (if applicable)
• I have read the CONTRIBUTING.md
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