chore: repo housekeeping (HK-1..HK-4, US-001 close-out)

[Hectormalvarez]

Summary

Repo cleanup per tasks/backlog.md housekeeping items, all verified against the code:

• HK-4: initialize memory-bank/ (6 files: projectbrief, productContext, systemPatterns, techContext, progress, activeContext)
• HK-1: README — drop build-time-generated robots.txt from the src/public/ tree (note added), add missing alarm storage test to the tests tree, add tasks/ to the repo-layout table
• HK-2: bump GitHub Actions off the deprecated Node 20 runtime — checkout@v7, setup-node@v7, setup-buildx@v4, login-action@v4, build-push-action@v7, hadolint-action@v3.5.0. All four Dockerfiles pre-validated locally with hadolint 2.15.1 + repo config
• HK-3: husky pre-commit guard — skip lint-staged unless staged files match its web/-scoped globs (silences the "could not find staged files" noise on docs/workflow-only commits; behavior verified across 3 commits)
• tasks/: US-001 closed out (PR feat(alarm): multiple alarms (US-001) #13 merged, Release Dev #5 deployed, healthz verified); HK-1..4 removed; HK-5 (nginx 1.27-alpine is an EOL mainline branch) added as backlog candidate

Test plan

• make check green (lint + typecheck + 135 tests)
• prettier clean on web/ and all changed root files
• hadolint 2.15.1 on all four Dockerfiles (config mounted per .hadolint.yaml comments)
• husky guard verified empirically on yml-only and md-only commits