docs: compact CLAUDE.md

[lesnik512]

Light CLAUDE.md compaction (template repo, no planning/ or architecture/).

• Replaced the verbatim Justfile recipe mirror in Common commands with a just --list / read-the-Justfile pointer plus the non-obvious "which-to-use-when" notes (test recreates the DB and passes args to pytest; run applies migrations; lint auto-fixes vs CI no-fix gate).
• Fixed a stale fact: the old just install line omitted --no-install-project, which the Justfile recipe actually uses.
• Kept all repo-specific guidance untouched: Stack, Architecture (request lifecycle, DI scopes, DB layer, settings, tests), and Conventions — this is a template, that guidance is the point. Verified the app/application.py:39 ty:ignore reference and CI description are still accurate.

Lint green: ruff format --check, ruff check --no-fix, ty check all pass.