ci: optimize workflows to cut redundant runs (#465)

* ci: optimize workflows to cut redundant runs and add timeout protection

Skip validation jobs (build, test, lint, Docker, E2E) on push-to-main
since code was already validated on the PR. Skip CI entirely for
release-please PRs (version bump only). Add timeout-minutes to all jobs
to prevent hung runners from burning 360 min (GitHub default). Add npm
caching to the start-app composite action. Add concurrency control to
docker-publish. Group Dependabot updates to batch PRs per ecosystem.

Estimated savings: ~10 min per release cycle, ~30-40 min/day.

* chore: add tsbuildinfo to gitignore

.gitignore

@@ -8,6 +8,7 @@ node_modules/
 
 # Build Outputs
 dist/
+*.tsbuildinfo
 
 # Sencho User Data & Mocks
 data/