ci: overhaul CI workflows for reliability and maintainability

- add CODEOWNERS assigning .github/ to @neonsolstice
- configure automated updates for npm, docker, and github-actions ecosystems
- replace custom change-detection bash script with dorny/paths-filter@v4
- consolidate lint, typecheck, and test into a single checks matrix job
- gate per-package change detection via has-changes step (matrix unavailable in job-level if)
- fix duplicate actions/setup-node call in setup-node-pnpm composite action
- scope pnpm store cache key by pnpm version to prevent stale cross-version cache hits
- pin all action references to commit SHAs
- fix container-image concurrency group to use commit SHA across all trigger types
- add timeout-minutes to all jobs
- document fail-fast: false rationale and checks: write trade-off

.gitignore

@@ -2,6 +2,7 @@ node_modules/
 dist/
 .env
 .env.local
+.env.prod
 client/.env
 server/.env
 *.local