Feature: modernize CI/CD with GitHub Actions

Replaced Concourse CI with GitHub Actions and upgraded to Go 1.25.

CI/CD Changes:
- Migrated from Concourse (ci/pipeline.yml) to GitHub Actions
- Added pipeline.yml with matrix strategy for parallel test/build
- Added code-check.yml for lint checks using native Go tools
- Added step summaries with test results and coverage reports
- Configured workflows for main branch and PR events

Go 1.25 Compatibility:
- Updated go.mod, Makefile, and workflows to Go 1.25
- Fixed format string errors in gplog calls with "%s" specifiers
- Fixed timeout test flakiness in cluster_test.go

Linting Improvements:
- Migrated .golangci.yml to v2 format (version, formats map)
- Simplified Makefile to use go install instead of Docker
- Fixed lint issues: errcheck, govet inline, unused symbols
- Aligned linter config with cloudberry-backup project conventions