Add a CI workflow that builds, tests, and lints both contracts

[priscaenoch]

Problem

This repository has no .github/workflows, so nothing verifies that the contracts build, pass tests, or lint on a pull request. The frontend and backend repositories both run CI; the contracts should too, especially given the non-obvious toolchain constraints documented in the README (soroban-sdk 21 rejects wasm reference-types, requiring the wasm-opt lowering step).

What needs to be done

• Add .github/workflows/ci.yml triggered on push and pull_request.
• Steps for both the explorer and ticket crates:
  • cargo fmt --check
  • cargo clippy -- -D warnings
  • cargo test
  • cargo build --release --target wasm32-unknown-unknown (install the target in CI)
• Optionally run wasm-opt from the deploy pipeline to confirm the MVP-lowering step still succeeds.

Files

• new .github/workflows/ci.yml

Acceptance deliverables

• CI runs on every PR and push and covers both crates.
• fmt, clippy, test, and wasm build failures block the check.

Tests to pass

• The workflow completes green on the current main.

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #1)
2. Your PR will be reviewed by the octraban maintainers

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