docs: add CLAUDE.md onboarding guide

[antosubash]

Summary

Adds a top-level CLAUDE.md so future Claude Code sessions can be productive without reverse-engineering the repo.

What's in it

• Commands: the make targets a session will actually invoke, including how to run a single pytest / vitest test.
• Architecture: the non-obvious pieces that require reading multiple files — entry-point discovery + topological sort, lifecycle hook ordering, LIFO middleware pipeline, per-module DB isolation (Postgres schema vs SQLite __tablename__ prefix), host-owned Alembic migrations with per-module branch_labels, and the Inertia page-key mapping (<ModuleName>/<PageName> → modules/<name>/<name>/pages/<PageName>.tsx).
• Conventions: SQLModel-everywhere (no Pydantic BaseModel or DeclarativeBase), the 300-line file cap, SM_<MODULE>_* settings convention, the framework→plugin import ban (SM009), the Zod-inside-useT() gotcha, and the ty rules deliberately disabled for SQLModel.
• Diagnostic codes (SM001–SM016) that show up in make doctor.
• Tests & fixtures: what conftest.py actually provides (db_session, client, authenticated_client with forged session cookie + CSRF).
• Pointers to docs/framework-conventions.md, docs/module-authoring.md, docs/e2e-testing.md, and docs/plans/ as source of truth rather than duplicating them.

Why

Without this, a fresh Claude Code session spends its first several turns re-discovering the module system, the SM* diagnostic codes, the LIFO middleware ordering, and the SQLModel-vs-Pydantic rule. The doc consolidates what's non-obvious and links to the authoritative references for everything else.

Test plan

• cat CLAUDE.md renders the expected sections
• Linked paths resolve (docs/framework-conventions.md, docs/module-authoring.md, docs/e2e-testing.md, docs/plans/)
• No source files changed — CI's non-doc checks should be unaffected