Add todo list contract example

[alexgduarte]

Summary

Adds an original simple-tier todo-list CosmWasm example.

The contract gives each account its own task sequence. Users can add tasks, complete them, reopen them, delete them, and query their own task list with pagination and optional completed-task filtering. The contract owner can update the maximum number of open tasks per user or transfer ownership.

What is included

• todo-list contract with instantiate, execute, query, state, and error modules
• README with example instantiate/execute/query messages
• Root README link under the introduction examples
• Six unit tests covering config, add/query, active-task limits, complete/reopen/delete behavior, pagination/filtering, and owner-only config updates

Verification

• cargo fmt
• cargo test
• git diff --check

Note: Cargo prints the same .cargo/config deprecation warning used by existing examples, but the tests pass.

Bounty / authorship

Related to #11 simple contract tier.

This is an original contract example authored for this PR. Contact for payout coordination: GitHub @alexgduarte; stablecoin payout details can be provided privately if accepted.