Document the staking model and its invariants

[mikewheeleer]

Document staking

Description

staking's model/invariants aren't documented. This issue records them.

Requirements and context

• Repository scope: Agentpay-Org/Agentpay-contracts only.
• Add docs/staking.md describing the staking model, entrypoints, and invariants.
• Cross-reference the code enforcing them.
• Keep accurate.

Suggested execution

• Fork the repo and create a branch
• git checkout -b docs/staking-91-model
• Implement changes
  • Write code in: the relevant module.
  • Write comprehensive tests in: cover the new behaviour and edge cases.
• Test and commit

Test and commit

• Run cargo fmt, cargo clippy --all-targets -- -D warnings, and cargo test.
• Cover edge cases: n/a — verify against source.
• Include the full test output in the PR description.

Example commit message

docs(staking): document model and invariants

Guidelines

• Minimum 95 percent test coverage for impacted modules.
• Clear, reviewer-focused documentation.
• Timeframe: 96 hours.

Community & contribution rewards

• 💬 Join the AgentPay community on Discord: https://discord.gg/eXvRKkgcv
• ⭐ This is a GrantFox OSS / Official Campaign task and may be rewarded. When your PR is merged you'll be prompted to rate the project — a 5-star rating is much appreciated.