chore: justfile recipes (lint, typecheck, test, architecture, check, frontend-check)

[constk]

Problem

Contributors should not need to memorise long uv run --frozen ... invocations. A justfile gives one canonical entry point per gate, identical locally and in CI.

Proposed solution

Port justfile from Teller. Recipes: dev (docker compose up), lint, fmt, typecheck, test, test-integration, coverage, architecture, pre-commit, eval, check (meta-recipe chaining lint typecheck architecture test), frontend-check. All use uv run --frozen to prevent silent re-resolve.

Acceptance criteria

• just (no args) lists every recipe with its description.
• just check runs lint+typecheck+architecture+test in order; exits 0 on empty scaffold.
• just frontend-check runs lint+format:check+check+test inside frontend/ (will be wired by feat: frontend scaffold (Vite + React 19.2 + TS strict, eslint flat + prettier + vitest) #21).
• Every recipe uses uv run --frozen (no implicit lock-file mutation).

Priority rationale

High: cuts onboarding friction for human + LLM contributors. The single just check is the pre-push gate.

Depends on

#2

[constk]

Auto-close did not fire from the squash-merge body. Marking closed retrospectively — the work shipped weeks ago and the project board's Done column reflects the actual state once the project-item move that follows this comment lands.