Update project conventions and introduce linting skill documentation

- Revised project-conventions.mdc to include a new section for running lint before tests, enhancing the development workflow.
- Added run-lint skill documentation in a new SKILL.md file, detailing usage for backend and frontend linting processes in the hnh-map monorepo.

.cursor/skills/run-lint/SKILL.md

@@ -0,0 +1,29 @@
+---
+name: run-lint
+description: Runs backend (golangci-lint) and frontend (ESLint) linters for the hnh-map monorepo. Use when the user asks to run the linter, lint, or to check code style and lint errors.
+---
+
+# Run linter
+
+## When to run
+
+- User asks to run the linter, run lint, or check lint/style.
+- After making code changes that should be validated by the project linters.
+
+## What to run
+
+Lint runs **in Docker** via the Makefile; no local Go or Node is required.
+
+From the repo root:
+
+- **Both backend and frontend:** `make lint` (runs golangci-lint then frontend ESLint in Docker).
+
+Uses `docker-compose.tools.yml`; the first run may build the backend-tools and frontend-tools images.
+
+## Scope
+
+- **Backend-only changes** (e.g. `internal/`, `cmd/`): `make lint` still runs both; backend lint runs first.
+- **Frontend-only changes** (e.g. `frontend-nuxt/`): `make lint` runs both; frontend lint runs second.
+- **Both or unclear**: run `make lint`.
+
+Report pass/fail and any linter errors or file/line references so the user can fix them.