fix(#190): add husky pre-commit + pre-push hooks with lint-staged

[Yzgaming005]

Closes #190

Summary

Adds Husky + lint-staged so formatting, linting, and tests run before code leaves the
developer's machine — instead of being caught only by CI minutes later.

Changes

Root package.json

• New devDependencies: husky ^9.1.7, lint-staged ^15.5.2
• scripts.prepare: "husky" → npm install wires the hooks automatically
• New convenience scripts: lint, format:check, test (workspace-aware)
• Top-level lint-staged config:
  • *.{ts,tsx,js,jsx,mjs,cjs} → prettier --write + eslint --fix --max-warnings=0
  • *.{json,md,yml,yaml} → prettier --write
  • *.rs → cargo fmt --manifest-path=apps/contracts/Cargo.toml --check

.husky/pre-commit (chmod +x)

npx lint-staged against staged files. Bypass with git commit --no-verify.

.husky/pre-push (chmod +x)

Runs npm test --workspace=<dir> only for workspaces touched by the commits being
pushed — so doc-only / config-only pushes don't block on unrelated test failures.

Overrides:

• HUSKY_PRE_PUSH_FORCE=1 → always run all workspaces
• HUSKY_PRE_PUSH_SKIP=1 → skip the hook entirely
• Bypass with git push --no-verify

CONTRIBUTING.md

• Documents npm install triggering prepare → husky
• Documents both hooks + the --no-verify escape hatch
• Notes that CI is the last line of defence, so don't bypass on shared branches without
  maintainer agreement

Verification

• ✅ npm install → husky installed under .husky/, both files chmod +x
• ✅ npx lint-staged --version → 15.5.2
• ✅ Pre-push diff-detect logic checked: only runs tests when apps/backend or
  apps/frontend files are in the commits being pushed
• ✅ Hooks bypassable via --no-verify (tested locally)
• ⚠️ Backend tests: 112/112 pass. Frontend tests have 6 pre-existing failures
  (lib/stellar-amount.spec.ts + app/components/BountyCard.spec.tsx) — these are
  unrelated to this change; the pre-push hook deliberately skips them when the frontend
  files aren't touched.

Acceptance criteria (from #190)

• Husky configured with pre-commit running lint-staged
• Pre-commit runs Prettier + ESLint on staged files (TS compile check runs via
  build script which CI already invokes)
• Pre-push runs unit tests for changed packages
• Hooks opt-in via --no-verify documented in CONTRIBUTING.md
• Setup documented in CONTRIBUTING.md