Add habit tracker contract example

[alexgduarte]

Summary

• Adds a new habit-tracker simple contract example.
• Stores one habit record per user address with current streak, best streak, total check-ins, last check-in height, and an optional short note.
• Demonstrates block-height cooldowns, streak reset logic, owner-only config updates, self-reset, and paginated record queries.
• Links the new example from the root README.

Validation

• cargo fmt --manifest-path habit-tracker/Cargo.toml -- --check
• cargo test --manifest-path habit-tracker/Cargo.toml --target-dir /private/tmp/codex-habit-tracker-target (8 passed)
• git diff --check

Bounty / Authorship

This is an original simple-tier contract example for bounty issue #11.

I authored this contract for this repository. Contact: GitHub @alexgduarte; stablecoin payout details can be provided privately if accepted.