feat: add linear-release-sync action

[sydorovdmytro]

Summary

• Add linear-release-sync composite action that syncs Linear issues to "Released" state when a GitHub release is published
• Finds PRs between releases, extracts Linear issue IDs from PR descriptions and branch names, and moves matching issues from "Ready for Release" to "Released"
• Supports optional filtering by Linear team and project names (comma-separated lists)
• Includes SHA-256 checksum verification for binary downloads, comprehensive unit tests across all packages, release and test CI workflows, and Renovate dependency grouping

Review fixes applied

• Added SHA-256 checksum verification for downloaded binary (supply-chain hardening)
• Fixed os.Kill → syscall.SIGTERM (uncatchable signal)
• Fixed nil-error wrapping in GraphQL mutations
• Moved logger from context.WithValue to LinearClient field (panic safety)
• Removed unused PageSize constant
• Added validation for required repo flag (previously silently failed on empty value)
• Disabled Go cache in release workflow (zizmor cache-poisoning finding)
• Bumped Go version from 1.22.5 to 1.26
• Renamed misleading integration_test.go to flow_test.go
• Added unit tests for changelog/pull-requests and changelog/releases packages

Test plan

• make test-linear-release-sync passes (65 tests)
• make build-linear-release-sync builds successfully
• make lint clean (actionlint + zizmor)
• CI workflow runs on PR
• Verify release workflow with dry-run after merge

Related to DEVOPS-607