Add CI workflow with functional testing

Implement GitHub Actions CI pipeline for testing Linmo across GNU and
LLVM RISC-V toolchains with automated reporting.

Features:
- Matrix testing for GNU and LLVM toolchains
- Automated app discovery and crash detection
- Functional test framework with criterion-based validation
- TOML-based result aggregation across toolchains
- Automated PR comments with detailed test results
- Proper failure handling and artifact collection

Components:
- .ci/ci-tools.sh: Unified tool for data collection, aggregation,
  and PR comment formatting
- .ci/run-app-tests.sh: Automated app testing with QEMU
- .ci/run-functional-tests.sh: Criterion-based functional tests
- .ci/setup-toolchain.sh: Automated toolchain installation
- .github/workflows/ci.yml: Complete CI workflow with summary job

The workflow ensures all tests run to completion, captures full output
for debugging, and fails CI based on the aggregated overall status.

Author: G36maid