Add maintainer onboarding documentation

docs/README.md

@@ -0,0 +1,39 @@
+# Documentation Index
+
+Start here when taking over the app:
+
+- [Onboarding](ONBOARDING.md): first-day setup, mental model, and where to look.
+- [Architecture](ARCHITECTURE.md): module boundaries, state flow, and design
+  decisions.
+- [Data Model](DATA_MODEL.md): persisted local state and important domain
+  models.
+- [Feature Map](FEATURE_MAP.md): product slices, canonical files, and common
+  extension points.
+- [Share Intake](SHARE_INTAKE.md): iOS/Android share entry, parsing, inbox, and
+  timestamp handling.
+- [AI Digest](AI_DIGEST.md): LLM providers, privacy model, prompts, digest
+  lifecycle, and review workflow.
+- [Storage And Migrations](STORAGE_AND_MIGRATIONS.md): Hive, legacy shared
+  preferences, schema versioning, and migration rules.
+- [Sync And Backend](SYNC_AND_BACKEND.md): fake/REST gateway boundary, sync
+  queue, OpenAPI contract, and repair flow.
+- [Implementation Playbook](IMPLEMENTATION_PLAYBOOK.md): example feature
+  implementation and files to update.
+- [Testing And Debugging](TESTING_AND_DEBUGGING.md): commands, fixtures,
+  diagnostics, and known test risks.
+- [Roadmap And Risks](ROADMAP_AND_RISKS.md): current gaps, priorities, and
+  traps for maintainers.
+
+Historical planning docs are still useful for context, but treat the files above
+as the current takeover documentation.
+
+Existing historical docs:
+
+- [Progress Log](progress.md)
+- [Open Tasks](open-tasks.md)
+- [Client Audit](client-audit-relationship-app.md)
+- [Client Implementation Plan](client-implementation-plan.md)
+- [Client Iteration Summary](client-iteration-summary.md)
+- [Backend REST/OpenAPI ADR](ADR/0002-backend-protocol-rest-openapi.md)
+- [Backendless Local LLM ADR](ADR/0003-backendless-local-llm-pipeline.md)
+- [OpenAPI Contract](api/openapi.yaml)