Add GitHub Actions CI pipeline (lint, test, build operator image)

[shreyanshjain7174]

Comprehensive CI: golangci-lint, unit + integration tests, helm lint, operator image build/push.

Acceptance:

• golangci-lint on every PR
• Unit tests for operator + agent runtime
• helm lint, helm template, kubeconform
• Image push to ghcr.io on merge to main
• Status badges in README

[shreyanshjain7174]

Verified as already implemented during M1 triage (2026-04-26). See evidence in the codebase.