docs(examples): add CosmWasm contract examples and deploy walkthrough

[Karry2019web]

Summary

This PR adds a complete examples/cosmwasm/ directory with four self-contained CosmWasm smart contract examples and a step-by-step walkthrough README, as described in issue #38.

Contracts included

1. counter/ — Classic Hello-World

• Increment, Decrement, Reset messages
• GetCount query
• Full unit tests with cw-multi-test
• Demonstrates: state management, messages, queries, testing

2. cw20-mintable/ — Minimal mintable CW20 token

• Wraps cw20-base with mint capability
• Full test coverage: initialization, mint, transfer, balance queries
• Demonstrates: CW20 standard, cw20-base integration

3. clock-counter/ — x/clock integration

• Implements SudoMsg::ClockEndBlock for EndBlock execution
• Manual increment for testing
• Demonstrates: x/clock registration, sudo entry point, chain-specific feature

4. cw-hooks-listener/ — x/cw-hooks integration (bonus)

• Listens for staking events (AfterValidatorCreated, AfterValidatorBonded, etc.)
• Stores events on-chain for querying
• Demonstrates: x/cw-hooks sudo messages, staking event handling

Walkthrough documentation

The top-level examples/cosmwasm/README.md includes:

• Prerequisites and setup instructions
• Quick-start guide with copy-pasteable safrochaind commands
• Step-by-step deploy walkthrough for the counter contract
• Troubleshooting guide

Acceptance criteria

• examples/cosmwasm/<name>/ directory per contract, each independently buildable
• Each contract has unit tests passing under cargo test
• Top-level examples/cosmwasm/README.md walks through the full local-node flow
• Examples reference usaf for fees and safro-testnet-1 as the chain ID
• Bonus: cw-hooks-listener demonstrates Safrochain-specific x/cw-hooks integration

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Closes #38