fix(agents): conservative default spending limits (#323)

[raymondidahor-bot]

Summary

Fixes #323

Freshly registered agents defaulted to effectively unlimited spending caps (10,000 USDC/tx, 100,000 USDC/day) with no way to tighten them. This PR makes the defaults restrictive named constants and documents them.

Changes

contract/agents/src/lib.rs

• Added named constants DEFAULT_MAX_PER_TX_STROOPS (500,000,000 stroops = 500 USDC) and DEFAULT_MAX_PER_DAY_STROOPS (5,000,000,000 stroops = 5,000 USDC) with a prominent comment block explaining the security rationale.
• register_agent now uses these constants instead of the inline permissive literals (10,000 USDC/tx, 100,000 USDC/day).
• Added a sanity validation in update_policy (max_per_tx_stroops > 0, max_per_day >= max_per_tx) so invalid limits are rejected at write time.

contract/agents/DEPLOY.md

• Added a "Default Spending Limits" section documenting the constants, the security rationale, and the exact stellar contract invoke command an owner uses to raise limits.

Acceptance criteria

• ✅ Defaults are conservative (500 USDC/tx, 5,000 USDC/day) and documented in DEPLOY.md
• ✅ Raising limits requires an explicit owner-authorized update_policy call (caller.require_auth() + agent.owner == caller)
• ✅ Default values are named constants, not inline literals

Close #323

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Co-Authored-By: Codebuff noreply@codebuff.com

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