Adopt Modular DDD enforced by tach

Context¶

The codebase was organized as Clean Architecture with technical layers at the project root (domain/, application/, infrastructure/, presentation/). docs/ARCHITECTURE_RULES.md calls instead for vertical bounded-context modules with machine-checked boundaries, so that adding a feature touches one module plus wiring and the tooling can prove it. Conventions without enforcement decay.

Decision¶

Restructure into config/, platform/ (shared technical capability), modules/<context>/ (each with api.py, CONTRACT.md, and domain / application / infra layers), and entry_points/ (thin wiring). The bounded contexts are documents, tags, and indexing. Enforce boundaries with tach: each module layer plus its api is a distinct tach module, dependencies are declared explicitly, cycles are forbidden, and tach check runs in CI alongside a contract-sync gate. Introduce the linter first over the old layout (permissive baseline), then restructure — per §12.

Consequences¶

• Easier: locating a capability; reasoning about a module from its CONTRACT.md without reading source; proving no illegal import was added.
• Harder: cross-cutting changes must respect the public api surface; adding a dependency requires declaring it in tach.toml.
• Now forbidden: importing another module's internals (only its api); import cycles; technical-layer directories at the project root.