feat(ci): implement automated build and test verification pipeline

[Demilade10]

Description

Set up a GitHub Actions workflow that runs cargo test and stellar contract build on every push and pull request, so build/test breakage is caught before merge instead of relying on manual verification.

Requirements and Context

• Right now all build/test verification is manual - this does not scale as contributors start opening PRs
• Must work on the wasm32v1-none build target
• soroban-sdk is pinned to 27.0.3 (not 27.0.5 - see Cargo.toml)
• crate-type must be ["rlib"] for cargo test to pass and ["cdylib", "rlib"] for stellar contract build to succeed
• CI runs on Linux, so Windows/MinGW-specific build quirks documented in CONTRIBUTING.md do not apply here

Suggested Execution

Branch name: chore/ci-pipeline

Files to touch:

• .github/workflows/ci.yml (new)
• README.md (add CI status badge)

Example commit message: chore(ci): add automated build and test verification pipeline

Test and Commit

• Workflow triggers on push and pull_request targeting main
• Installs the pinned Rust toolchain and the wasm32v1-none target
• Runs cargo test successfully
• Runs stellar contract build successfully
• Workflow fails the check if either step fails
• CI badge added to README and confirmed rendering
• Include a screenshot of a passing workflow run in the PR

Guidelines

• Assignment required before starting - comment to request assignment
• PR description must include: Closes feat(ci): implement automated build and test verification pipeline #1

Complexity

Medium - 150 points